2.1. Why would I want it?

Logical volume management is traditionally associated with large installations containing many disks but it is equally suited to small systems with a single disk or maybe two.

2.2. Benefits of Logical Volume Management on a Small System

One of the difficult decisions facing a new user installing Linux for the first time is how to partition the disk drive. The need to estimate just how much space is likely to be needed for system files and user files makes the installation more complex than is necessary and some users simply opt to put all their data into one large partition in an attempt to avoid the issue.

Once the user has guessed how much space is needed for /home /usr / (or has let the installation program do it) then is quite common for one of these partitions to fill up even if there is plenty of disk space in one of the other partitions.

With logical volume management, the whole disk would be allocated to a single volume group and logical volumes created to hold the / /usr and /home file systems. If, for example the /home logical volume later filled up but there was still space available on /usr then it would be possible to shrink /usr by a few megabytes and reallocate that space to /home.

Another alternative would be to allocate minimal amounts of space for each logical volume and leave some of the disk unallocated. Then, when the partitions start to fill up, they can be expanded as necessary.

As an example: Joe buys a PC with an 8.4 Gigabyte disk on it and installs Linux using the following partitioning system:

/boot /dev/hda1 10 Megabytes swap /dev/hda2 256 Megabytes / /dev/hda3 2 Gigabytes /home /dev/hda4 6 Gigabytes

This, he thinks, will maximize the amount of space available for all his MP3 files.

Sometime later Joe decides that he want to install the latest office suite and desktop UI available but realizes that the root partition isn't large enough. But, having archived all his MP3s onto a new writable DVD drive there is plenty of space on /home.

His options are not good:

1. Reformat the disk, change the partitioning scheme and reinstall.

2. Buy a new disk and figure out some new partitioning scheme that will require the minimum of data movement.

3. Set up a symlink farm on / pointing to /home and install the new software on /home

With LVM this becomes much easier:

Jane buys a similar PC but uses LVM to divide up the disk in a similar manner:

/boot     /dev/hda1        10 Megabytes
swap      /dev/vg00/swap   256 Megabytes
/         /dev/vg00/root     2 Gigabytes
/home     /dev/vg00/home     6 Gigabytes

         

boot is not included on the LV because bootloaders don't understand LVM volumes yet. It's possible boot on LVM will work, but you run the risk of having an unbootable system.

	root on LV should be used by advanced users only
	root on LVM requires an initrd image that activates the root LV. If a kernel is upgraded without building the necessary initrd image, that kernel will be unbootable. Newer distributions support lvm in their mkinitrd scripts as well as their packaged initrd images, so this becomes less of an issue over time.

When she hits a similar problem she can reduce the size of /home by a gigabyte and add that space to the root partition.

Suppose that Joe and Jane then manage to fill up the /home partition as well and decide to add a new 20 Gigabyte disk to their systems.

Joe formats the whole disk as one partition (/dev/hdb1) and moves his existing /home data onto it and uses the new disk as /home. But he has 6 gigabytes unused or has to use symlinks to make that disk appear as an extension of /home, say /home/joe/old-mp3s.

Jane simply adds the new disk to her existing volume group and extends her /home logical volume to include the new disk. Or, in fact, she could move the data from /home on the old disk to the new disk and then extend the existing root volume to cover all of the old disk.

2.3. Benefits of Logical Volume Management on a Large System

The benefits of logical volume management are more obvious on large systems with many disk drives.

Managing a large disk farm is a time-consuming job, made particularly complex if the system contains many disks of different sizes. Balancing the (often conflicting) storage requirements of various users can be a nightmare.

User groups can be allocated to volume groups and logical volumes and these can be grown as required. It is possible for the system administrator to "hold back" disk storage until it is required. It can then be added to the volume(user) group that has the most pressing need.

When new drives are added to the system, it is no longer necessary to move users files around to make the best use of the new storage; simply add the new disk into an existing volume group or groups and extend the logical volumes as necessary.

It is also easy to take old drives out of service by moving the data from them onto newer drives - this can be done online, without disrupting user service.

3.1. volume group (VG)

The Volume Group is the highest level abstraction used within the LVM. It gathers together a collection of Logical Volumes and Physical Volumes into one administrative unit.

3.2. physical volume (PV)

A physical volume is typically a hard disk, though it may well just be a device that 'looks' like a hard disk (eg. a software raid device).

3.3. logical volume (LV)

The equivalent of a disk partition in a non-LVM system. The LV is visible as a standard block device; as such the LV can contain a file system (eg. /home).

3.4. physical extent (PE)

Each physical volume is divided chunks of data, known as physical extents, these extents have the same size as the logical extents for the volume group.

3.5. logical extent (LE)

Each logical volume is split into chunks of data, known as logical extents. The extent size is the same for all logical volumes in the volume group.

3.6. Tying it all together

A concrete example will help:

Lets suppose we have a volume group called VG1, this volume group has a physical extent size of 4MB. Into this volume group we introduce 2 hard disk partitions, /dev/hda1 and /dev/hdb1. These partitions will become physical volumes PV1 and PV2 (more meaningful names can be given at the administrators discretion). The PV's are divided up into 4MB chunks, since this is the extent size for the volume group. The disks are different sizes and we get 99 extents in PV1 and 248 extents in PV2. We now can create ourselves a logical volume, this can be any size between 1 and 347 (248 + 99) extents. When the logical volume is created a mapping is defined between logical extents and physical extents, eg. logical extent 1 could map onto physical extent 51 of PV1, data written to the first 4 MB of the logical volume in fact be written to the 51st extent of PV1.

3.7. mapping modes (linear/striped)

The administrator can choose between a couple of general strategies for mapping logical extents onto physical extents:

1. Linear mapping will assign a range of PE's to an area of an LV in order eg., LE 1 - 99 map to PV1 and LE 100 - 347 map onto PV2.

2. Striped mapping will interleave the chunks of the logical extents across a number of physical volumes eg.,

   1st chunk of LE[1] -> PV1[1], 2nd chunk of LE[1] -> PV2[1], 3rd chunk of LE[1] -> PV3[1], 4th chunk of LE[1] -> PV1[2],

   and so on. In certain situations this strategy can improve the performance of the logical volume.

   	LVM 1 Caveat
   	LVs created using striping cannot be extended past the PVs they were originally created on in LVM 1.

   In LVM 2, striped LVs can be extended by concatenating another set of devices onto the end of the first set. So you can get into a situation where your LV is a 2 stripe set concatenated with a linear set concatenated with a 4 stripe set. Are you confused yet?

3.8. Snapshots

A wonderful facility provided by LVM is 'snapshots'. This allows the administrator to create a new block device which presents an exact copy of a logical volume, frozen at some point in time. Typically this would be used when some batch processing, a backup for instance, needs to be performed on the logical volume, but you don't want to halt a live system that is changing the data. When the snapshot device has been finished with the system administrator can just remove the device. This facility does require that the snapshot be made at a time when the data on the logical volume is in a consistent state - the VFS-lock patch for LVM1 makes sure that some filesystems do this automatically when a snapshot is created, and many of the filesystems in the 2.6 kernel do this automatically when a snapshot is created without patching.

	Full snapshot are automatically disabled
	If the snapshot logical volume becomes full it will be dropped (become unusable) so it is vitally important to allocate enough space. The amount of space necessary is dependent on the usage of the snapshot, so there is no set recipe to follow for this. If the snapshot size equals the origin size, it will never overflow.

LVM1 has read-only snapshots. Read-only snapshots work by creating an exception table, which is used to keep track of which blocks have been changed. If a block is to be changed on the origin, it is first copied to the snapshot, marked as copied in the exception table, and then the new data is written to the original volume.

In LVM2, snapshots are read/write by default. Read/write snapshots work like read-only snapshots, with the additional feature that if data is written to the snapshot, that block is marked in the exception table as used, and never gets copied from the original volume. This opens up many new possibilities that were not possible with LVM1's read-only snapshots. One example is to snapshot a volume, mount the snapshot, and try an experimental program that change files on that volume. If you don't like what it did, you can unmount the snapshot, remove it, and mount the original filesystem in its place. It is also useful for creating volumes for use with Xen. You can create a disk image, then snapshot it and modify the snapshot for a particular domU instance. You can then create another snapshot of the original volume, and modify that one for a different domU instance. Since the only storage used by a snapshot is blocks that were changed on the origin or the snapshot, the majority of the volume is shared by the domU's.

With the current LVM2/device-mapper code, the origin can be grown, but not shrunk. With LVM1, you cannot resize the origin.

	LVM 1 -> LVM 2 Upgrade Info
	Make sure to remove snapshot LVs before upgrading from LVM 1 to LVM 2. (See Section 4.1)

4.1. LVM 2 FAQ

4.1.1. I have LVM 1 installed and running on my system. How do I start using LVM 2?

4.1.2. Do I need a special lvm2 kernel module?

4.1.3. I get errors about /dev/mapper/control when I try to use the LVM 2 tools. What's going on?

4.1.4. Which commands and types of logical volumes are currently supported in LVM 2?

4.1.5. Does LVM 2 use a different format from LVM 1 for it's ondisk representation of Volume Groups and Logical Volumes?

4.1.6. Does LVM 2 support VGs and LVs created with LVM 1?

4.1.7. Can I upgrade my LVM 1 based VGs and LVs to LVM 2 native format?

4.1.8. I've upgraded to LVM 2, but the tools keep failing with out of memory errors. What gives?

4.1.9. I have my root partition on an LV in LVM 1. How do I upgrade to LVM 2? And what happened to lvmcreate_initrd?

4.1.10. How resilient is LVM to a sudden renumbering of physical hard disks?

4.1.11. I'm trying to fill my vg, and vgdisplay/vgs says that I have 1.87 GB free, but when I do an lvcreate vg -L1.87G it says "insufficient free extends". What's going on?

4.1.12. How are snapshots in LVM2 different from LVM1?

4.1.13. What is the maximum size of a single LV?

4.1.1. I have LVM 1 installed and running on my system. How do I start using LVM 2?

Here's the Quick Start instructions :)

1. Start by removing any snapshot LVs on the system. These are not handled by LVM 2 and will prevent the origin from being activated when LVM 2 comes up.

2. Make sure you have some way of booting the system other than from your standard boot partition. Have the LVM 1 tools, standard system tools (mount) and an LVM 1 compatible kernel on it in case you need to get back and fix some things.

3. Grab the LVM 2 tools source and the device mapper source and compile them. You need to install the device mapper library using "make install" before compiling the LVM 2 tools. Also copy the dm/scripts/devmap_mknod.sh script into /sbin. I recommend only installing the 'lvm' binary for now so you have access to the LVM 1 tools if you need them. If you have access to packages for LVM 2 and device-mapper, you can install those instead, but beware of them overwriting your LVM 1 tool set.

4. Get a device mapper compatible kernel, either built in or as a kernel module.

5. Figure out where LVM 1 was activated in your startup scripts. Make sure the device-mapper module is loaded by that point (if you are using device mapper as a module) and add '/sbin/devmap_mknod.sh; lvm vgscan; lvm vgchange -ay' afterward.

6. Install the kernel with device mapper support in it. Reboot. If all goes well, you should be running with lvm2.

4.1.2. Do I need a special lvm2 kernel module?

No. You need device-mapper. The lvm2 tools use device-mapper to interface with the kernel and do all their device mapping (hence the name device-mapper). As long as you have device-mapper, you should be able to use LVM2.

4.1.3. I get errors about /dev/mapper/control when I try to use the LVM 2 tools. What's going on?

The primary cause of this is not having run the "dmsetup mknodes" after rebooting into a dm capable kernel. This script generates the control node for device mapper.

If you don't have the "dmsetup mknodes", don't despair! (Though you should probably upgrade to the latest version of device-mapper.) It's pretty easy to create the /dev/mapper/control file on your own:

1. Make sure you have the device-mapper module loaded (if you didn't build it into your kernel).

2. Run

   # cat /proc/misc | grep device-mapper | awk '{print $1}'

   and note the number printed. (If you don't get any output, refer to step 1.)

3. Run

   # mkdir /dev/mapper

   - if you get an error saying /dev/mapper already exists, make sure it's a directory and move on.

4. Run

   # mknod /dev/mapper/control c 10 $number

   where $number is the number printed in step 2.

You should be all set now!

4.1.4. Which commands and types of logical volumes are currently supported in LVM 2?

If you are using the stable 2.4 device mapper patch from the lvm2 tarball, all the major functionality you'd expect using lvm1 is supported with the lvm2 tools. (You still need to remove snapshots before upgrading from lvm1 to lvm2)

If you are using the version of device mapper in the 2.6 kernel.org kernel series the following commands and LV types are not supported:

• pvmove

• snapshots

The beginnings of support for these features are in the unstable device mapper patches maintained by Joe Thornber.

4.1.5. Does LVM 2 use a different format from LVM 1 for it's ondisk representation of Volume Groups and Logical Volumes?

Yes. LVM 2 uses lvm 2 format metadata. This format is much more flexible than the LVM 1 format metadata, removing or reducing most of the limitations LVM 1 had.

4.1.6. Does LVM 2 support VGs and LVs created with LVM 1?

Yes. LVM 2 will activate and operate on VG and LVs created with LVM 1. The exception to this is snapshots created with LVM 1 - these should be removed before upgrading. Snapshots that remain after upgrading will have to be removed before their origins can be activated by LVM 2.

4.1.7. Can I upgrade my LVM 1 based VGs and LVs to LVM 2 native format?

Yes. Use vgconvert to convert your VG and all LVs contained within it to the new lvm 2 format metadata. Be warned that it's not always possible to revert back to lvm 1 format metadata.

4.1.8. I've upgraded to LVM 2, but the tools keep failing with out of memory errors. What gives?

One possible cause of this is that some versions of LVM 1 (The user that reported this bug originally was using Mandrake 9.2, but it is not necessarily limited to that distribution) did not put a UUID into the PV and VG structures as they were supposed to. The most current versions of the LVM 2 tools automatically fill UUIDs in for the structures if they see they are missing, so you should grab a more current version and your problem should be solved. If not, post to the linux-lvm mailing list

4.1.9. I have my root partition on an LV in LVM 1. How do I upgrade to LVM 2? And what happened to lvmcreate_initrd?

Upgrading to LVM 2 is a bit trickier with root on LVM, but it's not impossible. You need to queue up a kernel with device-mapper support and install the lvm2 tools (you might want to make a backup of the lvm 1 tools, or find a rescue disk with the lvm tools built in, in case you need them before you're done). Then find a mkinitrd script that has support for your distro and lvm 2.

Currently, this is the list of mkinitrd scripts that I know support lvm2, sorted by distro:

mkinitrd scripts with lvm 2 support

fedora

The latest fedora core 2 mkinitrd handles lvm2, but it relies on a statically built lvm binary from the latest lvm 2 tarball.

Redhat 9 users may be able to use this as well

Debian

There is an unofficial version here

Generic

There is a version in the lvm2 source tree under scripts/lvm2_createinitrd/. See the documentation in that directory for more details.

4.1.10. How resilient is LVM to a sudden renumbering of physical hard disks?

It's fine - LVM identifies PVs by UUID, not by device name.

Each disk (PV) is labeled with a UUID, which uniquely identifies it to the system. 'vgscan' identifies this after a new disk is added that changes your drive numbering. Most distros run vgscan in the lvm startup scripts to cope with this on reboot after a hardware addition. If you're doing a hot-add, you'll have to run this by hand I think. On the other hand, if your vg is activated and being used, the renumbering should not affect it at all. It's only the activation that needs the identifier, and the worst case scenario is that the activation will fail without a vgscan with a complaint about a missing PV.

The failure or removal of a drive that LVM is currently using will cause problems with current use and future activations of the VG that was using it.

4.1.11. I'm trying to fill my vg, and vgdisplay/vgs says that I have 1.87 GB free, but when I do an lvcreate vg -L1.87G it says "insufficient free extends". What's going on?

The 1.87 GB figure is rounded to 2 decimal places, so it's probably 1.866 GB or something. This is a human-readable output to give you a general idea of how big the VG is. If you want to specify an exact size, you must use extents instead of some multiple of bytes.

In the case of vgdisplay, use the Free PE count instead of the human readable capacity.

Free PE / Size 478 / 1.87 GB ^^^

So, this would indicate that you should do run

# lvcreate vg -l478

Note that instead of an upper-case 'L', we used a lower-case 'l' to tell lvm to use extents instead of bytes.

In the case of vgs, you need to instruct it to tell you how many extents are available:

# vgs -o +vg_free_count,vg_extent_count

This tell vgs to add the free extents and the total number of extents to the end of the vgs listing. Use the free extent number the same way you would in the above vgdisplay case.

4.1.12. How are snapshots in LVM2 different from LVM1?

In LVM2 snapshots are read/write by default, whereas in LVM1, snapshots were only read-only. See Section 3.8 for more details

4.1.13. What is the maximum size of a single LV?

The answer to this question depends upon the CPU architecture of your computer and the kernel you are a running:

• For 2.4 based kernels, the maximum LV size is 2TB. For some older kernels, however, the limit was 1TB due to signedness problems in the block layer. Red Hat Enterprise Linux 3 Update 5 has fixes to allow the full 2TB LVs. Consult your distribution for more information in this regard.

• For 32-bit CPUs on 2.6 kernels, the maximum LV size is 16TB.

• For 64-bit CPUs on 2.6 kernels, the maximum LV size is 8EB. (Yes, that is a very large number.)

4.2. LVM 1 FAQ

5.1. Download the source

• Device Mapper

• LVM 2

  Make sure you also grab the device mapper source

• LVM 1

The LVM 1 kernel patch must be generated using the LVM 1 source. More information regarding this can be found in Section 6.2

5.2. Download the development source via CVS

Note: the state of code in the CVS repository fluctuates wildly. It will contain bugs. Maybe ones that will crash LVM or the kernel. It may not even compile. Consider it alpha-quality code. You could lose data. You have been warned.

5.3. Before You Begin

To follow the development progress of LVM, subscribe to the LVM mailing lists, linux-lvm and the appropriate commit list (see Section C.1).

To build LVM from the CVS sources, you must have several GNU tools:

• the CVS client version 1.9 or better

• GCC 2.95.2

• GNU make 3.79

• autoconf, version 2.13 or better

5.4. Initial Setup

To make life easier in the future with regards to updating the CVS tree create the file $HOME/.cvsrc and insert the following lines. This configures useful defaults for the three most commonly used CVS commands. Do this now before proceeding any further.

diff -u -b -B
checkout -P
update -d -P

         

Also, if you are on a slow net link (like a dialup), you will want to add a line containing cvs -z5 in this file. This turns on a useful compression level for all CVS commands.

5.5. Checking Out Source Code

• Device Mapper library and tools

  The device mapper library is required to build LVM 2.

  The first time you download from cvs, you must login

  # cvs -d :pserver:cvs@sources.redhat.com:/cvs/dm login cvs

  The password is `cvs'. The command outputs nothing if successful and an error message if it fails. Only an initial login is required. All subsequent CVS commands read the password stored in the file $HOME/.cvspass for authentication.

  Use the following to check out a copy of the code

  # cvs -d :pserver:cvs@sources.redhat.com:/cvs/dm checkout device-mapper

  This will create a new directory device-mapper in your current directory containing the latest, up-to-the-minute device mapper code.

• LVM 2

  The first time you download from cvs, you must login

  # cvs -d :pserver:cvs@sources.redhat.com:/cvs/lvm2 login cvs

  The password is `cvs'. The command outputs nothing if successful and an error message if it fails. Only an initial login is required. All subsequent CVS commands read the password stored in the file $HOME/.cvspass for authentication.

  Use the following to check out a copy of the code

  # cvs -d :pserver:cvs@sources.redhat.com:/cvs/lvm2 checkout LVM2

  This will create a new directory LVM2 in your current directory containing the latest, up-to-the-minute LVM 2 code.

• LVM 1

  The first time you download from cvs, you must login

  # cvs -d :pserver:cvs@sources.redhat.com:/cvs/lvm login cvs

  The password is `cvs'. The command outputs nothing if successful and an error message if it fails. Only an initial login is required. All subsequent CVS commands read the password stored in the file $HOME/.cvspass for authentication.

  Use the following to check out a copy of the code

  # cvs -d :pserver:cvs@sources.redhat.com:/cvs/lvm checkout LVM

  This will create a new directory LVM in your current directory containing the latest, up-to-the-minute LVM 1 code.

CVS commands work from anywhere inside the source tree, and recurse downward. So if you happen to issue an update from inside the `tools' subdirectory it will work fine, but only update the tools directory and it's subdirectories. In the following command examples it is assumed that you are at the top of the source tree.

5.6. Code Updates

Code changes are made fairly frequently in the CVS repository. Announcements of this are automatically sent to the lvm-commit list.

You can update your copy of the sources to match the master repository with the update command. It is not necessary to check out a new copy. Using update is significantly faster and simpler, as it will download only patches instead of entire files and update only those files that have changed since your last update. It will automatically merge any changes in the CVS repository with any local changes you have made as well. Just cd to the directory you'd like to update and then type the following.

 # cvs update 
         

If you did not specify a tag when you checked out the source, this will update your sources to the latest version on the main branch. If you specified a branch tag, it will update to the latest version on that branch. If you specified a version tag, it will not do anything.

5.7. Starting a Project

Discuss your ideas on the developers list before you start. Someone may be working on the same thing you have in mind or they may have some good ideas about how to go about it.

5.8. Hacking the Code

So, have you found a bug you want to fix? Want to implement a feature from the TODO list? Got a new feature to implement? Hacking the code couldn't be easier. Just edit your copy of the sources. No need to copy files to .orig or anything. CVS has copies of the originals.

When you have your code in a working state and have tested as best you can with the hardware you have, generate a patch against the current sources in the CVS repository.

 # cvs update
 # cvs diff > patchfile
         

Mail the patch to the linux-lvm or dm-devel list (Section C.1) with a description of what changes or additions you implemented.

5.9. Conflicts

If someone else has been working on the same files as you have, you may find that there are conflicting modifications. You'll discover this when you try to update your sources.

 # cvs update 

 RCS file: LVM/tools/pvcreate.c,v
 retrieving revision 1.5
 retrieving revision 1.6
 Merging differences between 1.5 and 1.6 into pvcreate.c
 rcsmerge: warning: conflicts during merge
 cvs server: conflicts found in tools/pvcreate.c
 C tools/pvcreate.c

         

Don't panic! Your working file, as it existed before the update, is saved under the filename .#pvcreate.c.1.5. You can always recover it should things go horribly wrong. The file named `pvcreate.c' now contains both the old (i.e. your) version and new version of lines that conflicted. You simply edit the file and resolve each conflict by deleting the unwanted version of the lines involved.

 <<<<<<< pvcreate.c
    j++;
 =======
    j--;
 >>>>>>> 1.6

         

Don't forget to delete the lines with all the ``<'', ``='', and ``>'' symbols.

6.1. Building the device-mapper module

Device mapper is in 2.6.9 and later, so you just need to make sure it is enabled either as a module or builtin to the kernel. Look for /sys/class/misc/device-mapper or look in /proc/devices for a device-mapper entry to see if it is enabled. If neither are there, try modprobe dm_mod, then check again. For versions previous to 2.6.9, either you or your distro must patch the kernel to support it. Check the device mapper web page for more information.

6.2. Build the LVM 1 kernel module

To use LVM 1 you will have to build the LVM 1 kernel module (recommended), or if you prefer rebuild the kernel with the LVM 1 code statically linked into it.

Your Linux system is probably based on one of the popular distributions (eg., Red Hat, SuSE, Debian) in which case it is possible that you already have the LVM 1 module. Check the version of the tools you have on your system. You can do this by running any of the LVM command line tools with the '-h' flag. Use pvscan -h if you don't know any of the commands. If the version number listed at the top of the help listing is LVM 1.0.8, use your current setup and avoid the rest of this section.

 # modprobe lvm-mod 
         

7.1. Caldera

It is necessary to edit the file /etc/rc.d/rc.boot. Look for the line that says "Mounting local filesystems" and insert the vgscan and vgchange commands just before it.

You may also want to edit the the file /etc/rc.d/init.d/halt to deactivate the volume groups at shutdown. Insert the

vgchange -an

command near the end of this file just after the filesystems are unmounted or mounted read-only, before the comment that says "Now halt or reboot".

7.2. Debian

If you download the Debian lvm tool package, an initscript should be installed for you.

If you are installing LVM from source, you will still need to build your own initscript:

Create a startup script in /etc/init.d/lvm containing the following:

#!/bin/sh case "$1" in start) /sbin/vgscan /sbin/vgchange -ay ;; stop) /sbin/vgchange -an ;; restart|force-reload) ;; esac exit 0

Then execute the commands

# chmod 0755 /etc/init.d/lvm # update-rc.d lvm start 26 S . stop 82 1 .

Note the dots in the last command.

7.3. Mandrake

No initscript modifications should be necessary for current versions of Mandrake.

7.4. Redhat

For Redhat 7.0 and up, you should not need to modify any initscripts to enable LVM at boot time if LVM is built into the kernel. If LVM is built as a module, it may be necessary to modify /etc/rc.d/rc.sysinit to load the LVM module by adding "modprobe lvm-mod" before the section that reads:

# LVM initialization, take 2 (it could be on top of RAID) if [ -e /proc/lvm -a -x /sbin/vgchange -a -f /etc/lvmtab ]; then action $"Setting up Logical Volume Management:" /sbin/vgscan && /sbin/vgchange -a y fi

This init script fragment is from Red Hat 7.3 - other versions of Redhat may look slightly different.

For versions of Redhat older than 7.0, it is necessary to edit the file /etc/rc.d/rc.sysinit. Look for the line that says "Mount all other filesystems" and insert the vgscan and vgchange commands just before it. You should be sure that your root file system is mounted read/write before you run the LVM commands.

You may also want to edit the the file /etc/rc.d/init.d/halt to deactivate the volume groups at shutdown. Insert the

vgchange -an

command near the end of this file just after the filesystems are mounted read-only, before the comment that says "Now halt or reboot".

7.5. Slackware

Slackware 8.1 requires no updating of boot time scripts in order to make LVM work.

For versions previous to Slackware 8.1, you should apply the following patch to /etc/rc.d/rc.S

cd /etc/rc.d cp -a rc.S rc.S.old patch -p0 < rc.S.diff

(the cp part to make a backup in case).

----- snip snip file: rc.S.diff---------------
--- rc.S.or	Tue Jul 17 18:11:20 2001
+++ rc.S	Tue Jul 17 17:57:36 2001
@@ -4,6 +4,7 @@
 #
 # Mostly written by:  Patrick J. Volkerding, <volkerdi@slackware.com>
 #
+# Added LVM support <tgs@iafrica.com>

 PATH=/sbin:/usr/sbin:/bin:/usr/bin

@@ -28,19 +29,21 @@
   READWRITE=yes
 fi

+
 # Check the integrity of all filesystems
 if [ ! READWRITE = yes ]; then
-  /sbin/fsck -A -a
+  /sbin/fsck -a /
+  # Check only the root fs first, but no others
   # If there was a failure, drop into single-user mode.
   if [ ? -gt 1 ] ; then
     echo
     echo
-    echo "*******************************************************"
-    echo "*** An error occurred during the file system check. ***"
-    echo "*** You will now be given a chance to log into the  ***"
-    echo "*** system in single-user mode to fix the problem.  ***"
-    echo "*** Running 'e2fsck -v -y <partition>' might help.  ***"
-    echo "*******************************************************"
+    echo "************************************************************"
+    echo "*** An error occurred during the root file system check. ***"
+    echo "*** You will now be given a chance to log into the       ***"
+    echo "*** system in single-user mode to fix the problem.       ***"
+    echo "*** Running 'e2fsck -v -y <partition>' might help.       ***"
+    echo "************************************************************"
     echo
     echo "Once you exit the single-user shell, the system will reboot."
     echo
@@ -82,6 +85,44 @@
     echo -n "get into your machine and start looking for the problem. "
     read junk;
   fi
+  # okay / fs is clean, and mounted as rw
+  # This was an addition, limits vgscan to /proc thus
+  # speeding up the scan immensely.
+  /sbin/mount /proc
+
+  # Initialize Logical Volume Manager
+  /sbin/vgscan
+  /sbin/vgchange -ay
+
+  /sbin/fsck -A -a -R
+  #Check all the other filesystem, including the LVM's, excluding /
+
+  # If there was a failure, drop into single-user mode.
+  if [ ? -gt 1 ] ; then
+    echo
+    echo
+    echo "*******************************************************"
+    echo "*** An error occurred during the file system check. ***"
+    echo "*** You will now be given a chance to log into the  ***"
+    echo "*** system in single-user mode to fix the problem.  ***"
+    echo "*** Running 'e2fsck -v -y <partition>' might help.  ***"
+    echo "*** The root filesystem is ok and mounted readwrite ***"
+    echo "*******************************************************"
+    echo
+    echo "Once you exit the single-user shell, the system will reboot."
+    echo
+
+    PS1="(Repair filesystem) #"; export PS1
+    sulogin
+
+    echo "Unmounting file systems."
+    umount -a -r
+    mount -n -o remount,ro /
+    echo "Rebooting system."
+    sleep 2
+    reboot
+  fi
+
 else
   echo "Testing filesystem status: read-write filesystem"
   if cat /etc/fstab | grep ' / ' | grep umsdos 1> /dev/null 2> /dev/null ;
then
@@ -111,14 +152,16 @@
     echo -n "Press ENTER to continue. "
     read junk;
   fi
+
 fi

+
 # remove /etc/mtab* so that mount will create it with a root entry
 /bin/rm -f /etc/mtab* /etc/nologin /etc/shutdownpid

 # mount file systems in fstab (and create an entry for /)
 # but not NFS or SMB because TCP/IP is not yet configured
-/sbin/mount -a -v -t nonfs,nosmbfs
+/sbin/mount -a -v -t nonfs,nosmbfs,proc

 # Clean up temporary files on the /var volume:
 /bin/rm -f /var/run/utmp /var/run/*.pid /var/log/setup/tmp/*
--snip snip snip end of file---------------

         

7.6. SuSE

No changes should be necessary from 6.4 onward as LVM is included

9.1. Make LVM library and tools

Change into the LVM directory and do a ./configure followed by make. This will make all of the libraries and programs.

If the need arises you can change some options with the configure script. Do a ./configure --help to determine which options are supported. Most of the time this will not be necessary.

There should be no errors from the build process. If there are, see Reporting Errors and Bugs on how to report this.

You are welcome to fix them and send us the patches too. Patches are generally sent to the linux-lvm list.

9.2. Install LVM library and tools

After the LVM source compiles properly, simply run make install to install the LVM library and tools onto your system.

9.3. Removing LVM library and tools

To remove the library and tools you just installed, run make remove. You must have the original source tree you used to install LVM to use this feature.

10.1. Upgrading to LVM 1.0.8 with a non-LVM root partition

There are just a few simple steps to transition this setup, but it is still recommended that you backup your data before you try it. You have been warned.

1. Build LVM kernel and modules

   Follow the steps outlined in Chapter 5 - Section 6.2 for instructions on how to get and build the necessary kernel components of LVM.

2. Build the LVM user tools

   Follow the steps in Chapter 9 to build and install the user tools for LVM.

3. Setup your init scripts

   Make sure you have the proper init scripts setup as per Chapter 7.

4. Boot into the new kernel

   Make sure your boot-loader is setup to load the new LVM-enhanced kernel and, if you are using LVM modules, put an insmod lvm-mod into your startup script OR extend /etc/modules.conf (formerly /etc/conf.modules) by adding

   alias block-major-58 lvm-mod alias char-major-109 lvm-mod

   to enable modprobe to load the LVM module (don't forget to enable kmod).

   Reboot and enjoy.

10.2. Upgrading to LVM 1.0.8 with an LVM root partition and initrd

This is relatively straightforward if you follow the steps carefully. It is recommended you have a good backup and a suitable rescue disk handy just in case.

The "normal" way of running an LVM root file system is to have a single non-LVM partition called /boot which contains the kernel and initial RAM disk needed to start the system. The system I upgraded was as follows:

# df Filesystem 1k-blocks Used Available Use% Mounted on /dev/rootvg/root 253871 93384 147380 39% / /dev/hda1 17534 12944 3685 78% /boot /dev/rootvg/home 4128448 4568 3914168 0% /home /dev/rootvg/usr 1032088 332716 646944 34% /usr /dev/rootvg/var 253871 31760 209004 13% /var

/boot contains the old kernel and an initial RAM disk as well as the LILO boot files and the following entry in /etc/lilo.conf

# ls /boot System.map lost+found vmlinux-2.2.16lvm map module-info boot.0300 boot.b os2_d.b chain.b initrd.gz # tail /etc/lilo.conf image=/boot/vmlinux-2.2.16lvm label=lvm08 read-only root=/dev/rootvg/root initrd=/boot/initrd.gz append="ramdisk_size=8192"

1. Build LVM kernel and modules

   Follow the steps outlined in Chapter 5 - Section 6.2 for instructions on how to get and build the necessary kernel components of LVM.

2. Build the LVM user tools

   Follow the steps in Section 6.2 to build and install the user tools for LVM.

   Install the new tools. Once you have done this you cannot do any LVM manipulation as they are not compatible with the kernel you are currently running.

3. Rename the existing initrd.gz

   This is so it doesn't get overwritten by the new one

   # mv /boot/initrd.gz /boot/initrd08.gz

4. Edit /etc/lilo.conf

   Make the existing boot entry point to the renamed file. You will need to reboot using this if something goes wrong in the next reboot. The changed entry will look something like this:

   image=/boot/vmlinux-2.2.16lvm label=lvm08 read-only root=/dev/rootvg/root initrd=/boot/initrd08.gz append="ramdisk_size=8192"

5. Run lvmcreate_initrd to create a new initial RAM disk

   # lvmcreate_initrd 2.4.9

   Don't forget the put the new kernel version in there so that it picks up the correct modules.

6. Add a new entry into /etc/lilo.conf

   This new entry is to boot the new kernel with its new initrd.

   image=/boot/vmlinux-2.4.9lvm label=lvm10 read-only root=/dev/rootvg/root initrd=/boot/initrd.gz append="ramdisk_size=8192"

7. Re-run lilo

   This will install the new boot block

   # /sbin/lilo

8. Reboot

   When you get the LILO prompt select the new entry name (in this example lvm10) and your system should boot into Linux using the new LVM version.

   If the new kernel does not boot, then simply boot the old one and try to fix the problem. It may be that the new kernel does not have all the correct device drivers built into it, or that they are not available in the initrd. Remember that all device drivers (apart from LVM) needed to access the root device should be compiled into the kernel and not as modules.

   If you need to do any LVM manipulation when booted back into the old version, then simply recompile the old tools and install them with

   # make install

   If you do this, don't forget to install the new tools when you reboot into the new LVM version.

11.1. Initializing disks or disk partitions

Before you can use a disk or disk partition as a physical volume you will have to initialize it:

For entire disks:

• Run pvcreate on the disk:

  # pvcreate /dev/hdb

  This creates a volume group descriptor at the start of disk.

  	Not Recommended
  	Using the whole disk as a PV (as opposed to a partition spanning the whole disk) is not recommended because of the management issues it can create. Any other OS that looks at the disk will not recognize the LVM metadata and display the disk as being free, so it is likely it will be overwritten. LVM itself will work fine with whole disk PVs.

• If you get an error that LVM can't initialize a disk with a partition table on it, first make sure that the disk you are operating on is the correct one. If you are very sure that it is, run the following:

  	DANGEROUS
  	The following commands will destroy the partition table on the disk being operated on. Be very sure it is the correct disk.

  # dd if=/dev/zero of=/dev/diskname bs=1k count=1 # blockdev --rereadpt /dev/diskname

• When using LVM 1 on PCs with DOS partitions, set the partition type to 0x8e using fdisk or some other similar program. This step is unnecessary on PPC systems or when using LVM 2.

• Run pvcreate on the partition:

  # pvcreate /dev/hdb1

  This creates a volume group descriptor at the start of the /dev/hdb1 partition.

11.2. Creating a volume group

Use the 'vgcreate' program:

# vgcreate my_volume_group /dev/hda1 /dev/hdb1

NOTE: If you are using LVM 1 with devfs it is essential to use the full devfs name of the device rather than the symlinked name in /dev. So the above would be:

# vgcreate my_volume_group /dev/ide/host0/bus0/target0/lun0/part1 \ /dev/ide/host0/bus0/target1/lun0/part1

LVM 2 does not have this restriction.

You can also specify the extent size with this command if the default of 32MB is not suitable for you with the '-s' switch. In addition you can put some limits on the number of physical or logical volumes the volume can have.

11.3. Activating a volume group

After rebooting the system or running vgchange -an, you will not be able to access your VGs and LVs. To reactivate the volume group, run:

# vgchange -a y my_volume_group

11.4. Removing a volume group

Make sure that no logical volumes are present in the volume group, see later section for how to do this.

Deactivate the volume group:

# vgchange -a n my_volume_group

Now you actually remove the volume group:

# vgremove my_volume_group

11.5. Adding physical volumes to a volume group

Use 'vgextend' to add an initialized physical volume to an existing volume group.

# vgextend my_volume_group /dev/hdc1 ^^^^^^^^^ new physical volume

11.6. Removing physical volumes from a volume group

Make sure that the physical volume isn't used by any logical volumes by using then 'pvdisplay' command:

# pvdisplay /dev/hda1 --- Physical volume --- PV Name /dev/hda1 VG Name myvg PV Size 1.95 GB / NOT usable 4 MB [LVM: 122 KB] PV# 1 PV Status available Allocatable yes (but full) Cur LV 1 PE Size (KByte) 4096 Total PE 499 Free PE 0 Allocated PE 499 PV UUID Sd44tK-9IRw-SrMC-MOkn-76iP-iftz-OVSen7

If the physical volume is still used you will have to migrate the data to another physical volume using pvmove.

Then use 'vgreduce' to remove the physical volume:

# vgreduce my_volume_group /dev/hda1

11.7. Creating a logical volume

To create a 1500MB linear LV named 'testlv' and its block device special '/dev/testvg/testlv':

# lvcreate -L1500 -ntestlv testvg

To create a 100 LE large logical volume with 2 stripes and stripe size 4 KB.

# lvcreate -i2 -I4 -l100 -nanothertestlv testvg

If you want to create an LV that uses the entire VG, use vgdisplay to find the "Total PE" size, then use that when running lvcreate.

# vgdisplay testvg | grep "Total PE" Total PE 10230 # lvcreate -l 10230 testvg -n mylv

This will create an LV called mylv filling the testvg VG.

If you want the logical volume to be allocated from a specific physical volume in the volume group, specify the PV or PVs at the end of the lvcreate command line.

# lvcreate -L 1500 -ntestlv testvg /dev/sdg

11.8. Removing a logical volume

A logical volume must be closed before it can be removed:

# umount /dev/myvg/homevol # lvremove /dev/myvg/homevol lvremove -- do you really want to remove "/dev/myvg/homevol"? [y/n]: y lvremove -- doing automatic backup of volume group "myvg" lvremove -- logical volume "/dev/myvg/homevol" successfully removed

11.9. Extending a logical volume

To extend a logical volume you simply tell the lvextend command how much you want to increase the size. You can specify how much to grow the volume, or how large you want it to grow to:

# lvextend -L12G /dev/myvg/homevol lvextend -- extending logical volume "/dev/myvg/homevol" to 12 GB lvextend -- doing automatic backup of volume group "myvg" lvextend -- logical volume "/dev/myvg/homevol" successfully extended

will extend /dev/myvg/homevol to 12 Gigabytes.

# lvextend -L+1G /dev/myvg/homevol lvextend -- extending logical volume "/dev/myvg/homevol" to 13 GB lvextend -- doing automatic backup of volume group "myvg" lvextend -- logical volume "/dev/myvg/homevol" successfully extended

will add another gigabyte to /dev/myvg/homevol.

After you have extended the logical volume it is necessary to increase the file system size to match. how you do this depends on the file system you are using.

By default, most file system resizing tools will increase the size of the file system to be the size of the underlying logical volume so you don't need to worry about specifying the same size for each of the two commands.

1. ext2/ext3

   Unless you have patched your kernel with the ext2online patch it is necessary to unmount the file system before resizing it. (It seems that the online resizing patch is rather dangerous, so use at your own risk)

   # umount /dev/myvg/homevol/dev/myvg/homevol # resize2fs /dev/myvg/homevol # mount /dev/myvg/homevol /home

   If you don't have e2fsprogs 1.19 or later, you can download the ext2resize command from ext2resize.sourceforge.net and use that:

   # umount /dev/myvg/homevol/dev/myvg/homevol # ext2resize /dev/myvg/homevol # mount /dev/myvg/homevol /home

   For ext2 there is an easier way. LVM 1 ships with a utility called e2fsadm which does the lvextend and resize2fs for you (it can also do file system shrinking, see the next section).

   	LVM 2 Caveat
   	There is currently no e2fsadm equivalent for LVM 2 and the e2fsadm that ships with LVM 1 does not work with LVM 2.

   so the single command

   # e2fsadm -L+1G /dev/myvg/homevol

   is equivalent to the two commands:

   # lvextend -L+1G /dev/myvg/homevol # resize2fs /dev/myvg/homevol

   	Note
   	You will still need to unmount the file system before running e2fsadm.

2. reiserfs

   Reiserfs file systems can be resized when mounted or unmounted as you prefer:

   • Online:

     # resize_reiserfs -f /dev/myvg/homevol

   • Offline:

     # umount /dev/myvg/homevol # resize_reiserfs /dev/myvg/homevol # mount -treiserfs /dev/myvg/homevol /home

3. xfs

   XFS file systems must be mounted to be resized and the mount-point is specified rather than the device name.

   # xfs_growfs /home

4. jfs

   Just like XFS the JFS file system must be mounted to be resized and the mount-point is specified rather than the device name. You need at least Version 1.0.21 of the jfs-utils to do this.

   # mount -o remount,resize /home

   Known Kernel Bug

   Some kernel versions have problems with this syntax (2.6.0 is known to have this problem). In this case you have to explicitly specify the new size of the filesystem in blocks. This is extremely error prone as you must know the blocksize of your filesystem and calculate the new size based on those units. Example: If you were to resize a JFS file system to 4 gigabytes that has 4k blocks, you would write: # mount -o remount,resize=1048576 /home

11.10. Reducing a logical volume

Logical volumes can be reduced in size as well as increased. However, it is very important to remember to reduce the size of the file system or whatever is residing in the volume before shrinking the volume itself, otherwise you risk losing data.

1. ext2

   If you are using LVM 1 with ext2 as the file system then you can use the e2fsadm command mentioned earlier to take care of both the file system and volume resizing as follows:

   # umount /home # e2fsadm -L-1G /dev/myvg/homevol # mount /home

   	LVM 2 Caveat
   	There is currently no e2fsadm equivalent for LVM 2 and the e2fsadm that ships with LVM 1 does not work with LVM 2.

   If you prefer to do this manually you must know the new size of the volume in blocks and use the following commands:

   # umount /home # resize2fs /dev/myvg/homevol 524288 # lvreduce -L-1G /dev/myvg/homevol # mount /home

2. reiserfs

   Reiserfs seems to prefer to be unmounted when shrinking

   # umount /home # resize_reiserfs -s-1G /dev/myvg/homevol # lvreduce -L-1G /dev/myvg/homevol # mount -treiserfs /dev/myvg/homevol /home

3. xfs

   There is no way to shrink XFS file systems.

4. jfs

   There is no way to shrink JFS file systems.

11.11. Migrating data off of a physical volume

To take a disk out of service it must first have all of its active physical extents moved to one or more of the remaining disks in the volume group. There must be enough free physical extents in the remaining PVs to hold the extents to be copied from the old disk. For further detail see Section 13.5.

12.1. Multiple partitions on the same disk

LVM allows you to create PVs (physical volumes) out of almost any block device so, for example, the following are all valid commands and will work quite happily in an LVM environment:

# pvcreate /dev/sda1 # pvcreate /dev/sdf # pvcreate /dev/hda8 # pvcreate /dev/hda6 # pvcreate /dev/md1

In a "normal" production system it is recommended that only one PV exists on a single real disk, for the following reasons:

• Administrative convenience

  It's easier to keep track of the hardware in a system if each real disk only appears once. This becomes particularly true if a disk fails.

• To avoid striping performance problems

  LVM can't tell that two PVs are on the same physical disk, so if you create a striped LV then the stripes could be on different partitions on the same disk resulting in a decrease in performance rather than an increase.

• Migration of existing system to LVM

  On a system with few disks it may be necessary to move data around partitions to do the conversion (see Section 13.8)

• Splitting one big disk between Volume Groups

  If you have a very large disk and want to have more than one volume group for administrative purposes then it is necessary to partition the drive into more than one area.

If you do have a disk with more than one partition and both of those partitions are in the same volume group, take care to specify which partitions are to be included in a logical volume when creating striped volumes.

The recommended method of partitioning a disk is to create a single partition that covers the whole disk. This avoids any nasty accidents with whole disk drive device nodes and prevents the kernel warning about unknown partition types at boot-up.

12.2. Sun disk labels

You need to be especially careful on SPARC systems where the disks have Sun disk labels on them.

The normal layout for a Sun disk label is for the first partition to start at block zero of the disk, thus the first partition also covers the area containing the disk label itself. This works fine for ext2 filesystems (and is essential for booting using SILO) but such partitions should not be used for LVM. This is because LVM starts writing at the very start of the device and will overwrite the disk label.

If you want to use a disk with a Sun disk label with LVM, make sure that the partition you are going to use starts at cylinder 1 or higher.

13.1. Setting up LVM on three SCSI disks

For this recipe, the setup has three SCSI disks that will be put into a logical volume using LVM. The disks are at /dev/sda, /dev/sdb, and /dev/sdc.

---

13.1.1. Preparing the disks

Before you can use a disk in a volume group you will have to prepare it:

	Warning!
	The following will destroy any data on /dev/sda, /dev/sdb, and /dev/sdc

Run pvcreate on the disks

# pvcreate /dev/sda # pvcreate /dev/sdb # pvcreate /dev/sdc

This creates a volume group descriptor area (VGDA) at the start of the disks.

13.2. Setting up LVM on three SCSI disks with striping

For this recipe, the setup has three SCSI disks that will be put into a logical volume using LVM. The disks are at /dev/sda, /dev/sdb, and /dev/sdc.

	Note
	It is not currently possible to add a disk to a striped logical volume in LVM 1. Use LVM 2 with the lvm 2 format metadata if you wish to be able to do so able to do so.

---

13.2.1. Preparing the disk partitions

Before you can use a disk in a volume group you will have to prepare it:

	Warning!
	The following will destroy any data on /dev/sda, /dev/sdb, and /dev/sdc

Run pvcreate on the disks:

# pvcreate /dev/sda # pvcreate /dev/sdb # pvcreate /dev/sdc

This creates a volume group descriptor area (VGDA) at the start of the disks.

---

13.2.3. Creating the Logical Volume

If the volume group looks correct, it is time to create a logical volume on top of the volume group.

You can make the logical volume any size you like (up to the size of the VG you are creating it on; it is similar to a partition on a non LVM setup). For this example we will create just a single logical volume of size 1GB on the volume group. The logical volume will be a striped set using for the 4k stripe size. This should increase the performance of the logical volume.

# lvcreate -i3 -I4 -L1G -nmy_logical_volume my_volume_group lvcreate -- rounding 1048576 KB to stripe boundary size 1056768 KB / 258 PE lvcreate -- doing automatic backup of "my_volume_group" lvcreate -- logical volume "/dev/my_volume_group/my_logical_volume" successfully created

	Note
	If you create the logical volume with a '-i2' you will only use two of the disks in your volume group. This is useful if you want to create two logical volumes out of the same physical volume, but we will not touch that in this recipe.

13.3. Add a new disk to a multi-disk SCSI system

13.4. Taking a Backup Using Snapshots

Following on from the previous example we now want to use the extra space in the "ops" volume group to make a database backup every evening. To ensure that the data that goes onto the tape is consistent we use an LVM snapshot logical volume.

A snapshot volume is a special type of volume that presents all the data that was in the volume at the time the snapshot was created. For a more detailed description, see Section 3.8, Snapshots. This means we can back up that volume without having to worry about data being changed while the backup is going on, and we don't have to take the database volume offline while the backup is taking place.

In LVM1, this type of volume was read-only, but LVM2 creates read/write snapshots by default.

---

13.4.1. Create the snapshot volume

There is a little over 500 Megabytes of free space in the "ops" volume group, so we will use all of it to allocate space for the snapshot logical volume. A snapshot volume can be as large or a small as you like but it must be large enough to hold all the changes that are likely to happen to the original volume during the lifetime of the snapshot. So here, allowing 500 megabytes of changes to the database volume which should be plenty.

# lvcreate -L592M -s -n dbbackup /dev/ops/databases lvcreate -- WARNING: the snapshot must be disabled if it gets full lvcreate -- INFO: using default snapshot chunk size of 64 KB for "/dev/ops/dbbackup" lvcreate -- doing automatic backup of "ops" lvcreate -- logical volume "/dev/ops/dbbackup" successfully created

	Full snapshot are automatically disabled
	If the snapshot logical volume becomes full it will be dropped (become unusable) so it is vitally important to allocate enough space. The amount of space necessary is dependent on the usage of the snapshot, so there is no set recipe to follow for this. If the snapshot size equals the origin size, it will never overflow.

13.5. Removing an Old Disk

Say you have an old IDE drive on /dev/hdb. You want to remove that old disk but a lot of files are on it.

	Backup Your System
	You should always backup your system before attempting a pvmove operation.

---

13.5.1. Distributing Old Extents to Existing Disks in Volume Group

If you have enough free extents on the other disks in the volume group, you have it easy. Simply run

# pvmove /dev/hdb pvmove -- moving physical extents in active volume group "dev" pvmove -- WARNING: moving of active logical volumes may cause data loss! pvmove -- do you want to continue? [y/n] y pvmove -- 249 extents of physical volume "/dev/hdb" successfully moved

This will move the allocated physical extents from /dev/hdb onto the rest of the disks in the volume group.

	pvmove is Slow
	Be aware that pvmove is quite slow as it has to copy the contents of a disk block by block to one or more disks. If you want more steady status reports from pvmove, use the -v flag.

---

13.5.1.1. Remove the unused disk

We can now remove the old IDE disk from the volume group.

# vgreduce dev /dev/hdb vgreduce -- doing automatic backup of volume group "dev" vgreduce -- volume group "dev" successfully reduced by physical volume: vgreduce -- /dev/hdb

The drive can now be either physically removed when the machine is next powered down or reallocated to other users.

13.6. Moving a volume group to another system

It is quite easy to move a whole volume group to another system if, for example, a user department acquires a new server. To do this we use the vgexport and vgimport commands.

vgexport/vgimport is not necessary to move drives from one system to another. It is an administrative policy tool to prevent access to volumes in the time it takes to move them.

13.7. Splitting a volume group

There is a new group of users "design" to add to the system. One way of dealing with this is to create a new volume group to hold their data. There are no new disks but there is plenty of free space on the existing disks that can be reallocated.

# mke2fs /dev/design/users
mke2fs 1.18, 11-Nov-1999 for EXT2 FS 0.5b, 95/08/09
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
96384 inodes, 192512 blocks
9625 blocks (5.00<!-- ) reserved for the super user
First data block=0
6 block groups
32768 blocks per group, 32768 fragments per group
16064 inodes per group
Superblock backups stored on blocks: 
        32768, 98304, 163840

Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done
        

# mkdir -p /mnt/design/users mount /dev/design/users /mnt/design/users/
        

13.8. Converting a root filesystem to LVM 1

	Backup Your System
	It is strongly recommended that you take a full backup of your system before attempting to convert to root on LVM 1.

	Upgrade Complications
	Having your root filesystem on LVM 1 can significantly complicate upgrade procedures (depending on your distribution) so it should not be attempted lightly. Particularly, you must consider how you will insure that the LVM 1 kernel module (if you do not have LVM 1 compiled into the kernel) as well as the vgscan/vgchange tools are available before, during, and after the upgrade.

Recovery Complications

Having your root filesystem on LVM 1 can significantly complicate recovery of damaged filesystems. If you lose your initrd, it will be very difficult to boot your system. You will need to have a recover disk that contains the kernel, LVM 1 module, and LVM 1 tools, as well as any tools necessary to recover a damaged filesystem. Be sure to make regular backups and have an up-to-date alternative boot method that allows for recovery of LVM 1.

In this example the whole system was installed in a single root partition with the exception of /boot. The system had a 2 gig disk partitioned as:

/dev/hda1 /boot /dev/hda2 swap /dev/hda3 /

The / partition covered all of the disk not used by /boot and swap. An important prerequisite of this procedure is that the root partition is less that half full (so that a copy of it can be created in a logical volume). If this is not the case then a second disk drive should be used. The procedure in that case is similar but there is no need to shrink the existing root partition and /dev/hda4 should be replaced with (eg) /dev/hdb1 in the examples.

To do this it is easiest to use GNU parted. This software allows you to grow and shrink partitions that contain filesystems. It is possible to use resize2fs and fdisk to do this but GNU parted makes it much less prone to error. It may be included in your distribution, if not you can download it from ftp://ftp.gnu.org/pub/gnu/parted.

Once you have parted on your system AND YOU HAVE BACKED THE SYSTEM UP:

# lvmcreate_initrd
        

# lilo
        

13.9. Recover physical volume metadata

If you get the warning "incorrect metadata area header checksum" or something about not being able to find PV with UUID foo, you probably toasted the volume group descriptor area and lvm startup can't occur.

	Only run on non-functional VG
	Don't do this on a properly working lvm. You need to specify the correct physical volume to pvcreate or you may lose your data.

Extract the exact uuid for the PV that was overwritten from the file /etc/lvm/archive/VolumeGroupName_XXXXX.vg. (Where XXXXX represents the number of the last known good archived lvm metadata).

Use pvcreate to restore the metadata: pvcreate --uuid "<some_long_string>" --restorefile /etc/lvm/archive/VolumeGroupName_XXXXX.vg <PhysicalVolume>

If you are lucky you'll find that the on-disk lvm metadata takes at least so much space as what it was overwritten with. The above command has been know to recover a PV overwritten with mkswap. If whatever overwrote the VGDA writes past that area, LVs may be affected. In this case, fsck might be able to fix the filesystem on the LV, or you may need more drastic measures to pull data off of it. Contact your local friendly filesystem expert for help in that case.

pvcreate only overwrites the lvm metadata areas on disk and doesn't touch the data areas (the logical volumes).