LVM 2 FAQ

LVM HOWTO
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# 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.)

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