Difference between revisions of "LVM"

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m (Create Logical Volumes: a little more restructuring)
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  # lvextend -L 20G VolGroup00/lvolhome (or lvresize -L +5G VolGroup00/lvolhome)
 
  # lvextend -L 20G VolGroup00/lvolhome (or lvresize -L +5G VolGroup00/lvolhome)
 
  # resize2fs /dev/VolGroup00/lvolhome
 
  # resize2fs /dev/VolGroup00/lvolhome
You may use lvresize instead of lvextend.
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You may use {{Codeline|lvresize}} instead of {{Codeline|lvextend}}.
  
'''Attention''': Not all filesystems support growing without loss of data and/or growing online.
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If you want to fill all the free space on a volume group, use the next command:
 +
# lvextend -l +100%FREE VolGroup00/lvolhome
  
'''Information''': If you don't resize your filesystem, you will still have a volume with the same size as before (volume will be bigger but partly unused).
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{{Warning|Not all filesystems support growing without loss of data and/or growing online.}}
  
'''Hint''': If you want to fill all the free space on a volume group use the next command:
+
{{Note|If you don't resize your filesystem, you will still have a volume with the same size as before (volume will be bigger but partly unused).}}
# lvextend -l +100%FREE VolGroup00/lvolhome
+
  
 
== Shrink logical volume ==
 
== Shrink logical volume ==

Revision as of 03:06, 29 August 2010

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Introduction

LVM is a Logical Volume Manager for the Linux kernel. With LVM you can abstract your storage space and have "virtual partitions" which are easier to modify. The basic building blocks of LVM are:

  • Physical volume (PV): Partition on hard disk (or even hard disk itself or loopback file) on which you can have volume groups. It has a special header and is divided into physical extents. Think of physical volumes as big building blocks which can be used to build your hard drive.
  • Volume group (VG): Group of physical volumes that are used as storage volume (as one disk). They contain logical volumes. Think of volume groups as hard drives.
  • Logical volume(LV): A "virtual/logical partition" that resides in a volume group and is composed of physical extents. Think of logical volumes as normal partitions.
  • Physical extent (PE): A small part of a disk (usually 4MB) that can be assigned to a logical Volume. Think of physical extents as parts of disks that can be allocated to any partition.

With LVM you can more easily handle your partitions (logical volumes) than normal hard drive partitions. For example, you can:

  • Use any number of disks as one big disk(VG)
  • Have partitions(LV) streched over several disks (they can be as big as all of your disk storage together)
  • Resize/create/delete partitions(LV) and disks(VG) as you like (it doesn't depend on position of the logical volumes within volume groups as with normal partitions)
  • Resize/create/delete partitions(LV) and disks(VG) online (filesystems on them still need to be resized, but some support online resizing)
  • Name your disks(VG) and partitions(LV) as you like
  • Create small partitions(LV) and resize them "dynamically" as they get more filled (growing must be still done by hand, but you can do it online with some filesystems)
  • ...

Example:

Physical disks
                
  Disk1 (/dev/sda):
     _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    |Partition1 50GB (Physical volume) |Partition2 80GB (Physical volume)     |
    |/dev/sda1                         |/dev/sda2                             |
    |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
                                  
  Disk2 (/dev/sdb):
     _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    |Partition1 120GB (Physical volume)                 |
    |/dev/sdb1                                          |
    | _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _|
LVM logical volumes

  Volume Group1 (/dev/MyStorage/ = /dev/sda1 + /dev/sda2 + /dev/sdb1):
     _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
    |Logical volume1 15GB  |Logical volume2 35GB      |Logical volume3 200GB               |
    |/dev/MyStorage/rootvol|/dev/MyStorage/homevol    |/dev/MyStorage/mediavol             |
    |_ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |

To sum it all up: With LVM you can use all your storage space as one big disk (volume group) and have more flexibility over your partitions (logical volumes).

Installation

Before doing anything we need to load the appropriate module:

# modprobe dm-mod

If you already have Arch Linux installed and you just want to add/try a partition with LVM jump to partition disks.

Installing Arch Linux on LVM

Prior to running Arch Linux install scripts (/arch/setup) to install Arch Linux, you need to partition your disk with cfdisk (or any other tool of your liking). Because grub legacy (grub with version less than 1.0) can't boot from LVM logical volumes you can't have /boot in LVM, so you need to create a boot partition. 100MB should be enough. The other solution would be to use lilo or grub with version 1.95 or newer.

Partition disks

Next you need to create a partition for LVM. Its filesystem type should be 'Linux LVM', so use a partition id 0x8e (filesystem type: 8e). You need to create only one LVM partition on each disk you want to use with LVM. Your logical volumes will reside inside these partitions so size them accordingly. If you will use only LVM and no other external partitions, use all the free space on each disk.

Warning: /boot can't reside in an LVM partition because grub (version<1.95) does not support LVM.
Tip: All LVM partitions on all disks can be configured to appear as one big disk.

Create Physical volumes

Now you need to initialize these partitions so they can be used by LVM. Use 'fdisk -l' to find out which partitions have filesystem type 'Linux LVM' and create a physical volume on them:

# pvcreate /dev/sda2

Substitute /dev/sda2 with all your partitions to create physical volumes on all of them. This command creates a header on each partition so it can be used for LVM. You can track created physical volumes with:

# pvdisplay

Create Volume group(s)

Next step is to create a volume group on this physical volume. First you need to create a volume group on one of the new partitions and then add to it all other physical volumes you want to have in it:

# vgcreate VolGroup00 /dev/sda2
# vgextend VolGroup00 /dev/sdb1

Also you can use any other name you like instead of VolGroup00 for a volume group when creating it. You can track how your volume group grows with:

# vgdisplay
Note: You can create more than one volume group if you need to, but then you won't have all your storage presented as one disk.

Create Logical Volumes

Now we need to create logical volumes on this volume group. You create a logical volume with the next command by giving the name of a new logical volume, its size, and the volume group it will live on:

# lvcreate -L 10G VolGroup00 -n lvolhome

This will create a logical volume that you can access later with Template:Filename or Template:Filename. Same as with the volume groups, you can use any name you want for your logical volume when creating it.

To create swap on a logical volume, an additional argument is needed:

# lvcreate -C y -L 10G VolGroup00 -n lvolswap

The Template:Codeline is used to create a contiguous partition, which means that your swap space doesn't get partitioned over one or more disks nor over non-contiguous physical extents.

If you want to fill all the free space left on a volume group, use the next command:

# lvcreate -l +100%FREE VolGroup00 -n lvolmedia

You can track created logical volumes with:

# lvdisplay
Note: You may need to load the device-mapper kernel module (modprobe dm-mod) for the above commands to succeed:
Tip: You can start out with relatively small logical volumes and expand them later if needed. For simplicity, leave some free space in the volume group so there is room for expansion.

Create filesystem and mount logical volumes

Your logical volumes should now be located in /dev/mapper/ and /dev/YourVolumeGroupName. If you can't find them use the next commands to bring up the module for creating device nodes and to make volume groups availabile:

# modprobe dm-mod
# vgchange -ay

Now you can create filesystems on logical volumes and mount them as normal partitions (if you are installing Arch linux, skip this step. Use the arch installer to pick the lvm partitions that you have created):

# mkfs.ext3 /dev/mapper/VolGroup00-lvolhome
# mount /dev/mapper/VolGroup00-lvolhome /home

If you are installing Arch linux, start /arch/setup, go to Prepare Hard Drive directly to step 3 Set Filesystem Mountpoints and read the Important section below before proceeding with installation!

Important

There are just a few things you need to be careful while using/installing Arch Linux with LVM (in brackets are the corresponding menus during installation):

  1. When choosing mountpoints, just select your newly created logical volumes (use: /dev/mapper/Volgroup00-lvolhome).
    Do NOT select the actual partitions on which logical volumes were created (don't use: /dev/sda2). (Set Filesystem Mountpoints)
  2. Make sure you change USELVM="no" to USELVM="yes" in /etc/rc.conf (Configure System)
  3. Make sure that lvm2 is in the HOOKS section of /etc/mkinitcpio.conf just before the filesystems so that your kernel will find LVM volumes at boot time. Also, if you wish to use LVM snapshots, add dm-snapshot to the MODULES variable. (Configure System)
  4. Make sure /boot/grub/menu.lst uses the right volumes for root. It should look something like this: (Install Bootloader)
...
# (0) Arch Linux
title  Arch Linux
root   (hd0,0)
kernel /vmlinuz26 root=/dev/mapper/VolGroup00-lvolroot resume=/dev/mapper/VolGroup00-lvolswap ro
initrd /kernel26.img
...

If you are using LILO check /etc/lilo.conf:

image=/boot/vmlinuz26
       label=arch
       append="root=/dev/mapper/VolGroup00-lvolroot resume=/dev/mapper/VolGroup00-lvolswap ro"
       initrd=/boot/kernel26.img

Configuration

Grow logical volume

To grow a logical volume you first need to grow the logical volume and then the filesystem to use the newly created free space. Let's say we have a logical volume of 15GB with ext3 on it and we want to grow it to 20G. We need to do the following steps:

# lvextend -L 20G VolGroup00/lvolhome (or lvresize -L +5G VolGroup00/lvolhome)
# resize2fs /dev/VolGroup00/lvolhome

You may use Template:Codeline instead of Template:Codeline.

If you want to fill all the free space on a volume group, use the next command:

# lvextend -l +100%FREE VolGroup00/lvolhome
Warning: Not all filesystems support growing without loss of data and/or growing online.
Note: If you don't resize your filesystem, you will still have a volume with the same size as before (volume will be bigger but partly unused).

Shrink logical volume

Because your filesystem is probably as big as the logical volume it resides on, you need to shrink the filesystem first and then shrink the logical volume. Depending on your filesystem, you may need to unmount it first. Let's say we have a logical volume of 15GB with ext3 on it and we want to shrink it to 10G. We need to do the following steps:

# resize2fs /dev/VolGroup00/lvolhome 9G
# lvreduce -L 10G VolGroup00/lvolhome (or lvresize -L -5G VolGroup00/lvolhome)
# resize2fs /dev/VolGroup00/lvolhome

Here we shrunk the filesystem more than needed so that when we shrunk the logical volume we didn't accidentally cut of the end of the filesystem. After that we normally grow the filesystem to fill all free space left on logical volume. You may use lvresize instead of lvreduce.

Attention: Don't reduce the filesystem size to less than it is used by data on it or you can lose your data.

Attention: Not all filesystems support shrinking without loss of data and/or shrinking online.

Attention: It is better to reduce the filesystem to lower size than the logical volume, so that after a resizing logical volume, we don't accidentally cut off some data from the end of the filesystem.

Add partition to a volume group

To add a partition to your volume group you must first make its type 'Linux LVM' (for example with cfdisk). Then you need to create a physical volume on it and extend the volume group over it:

# pvcreate /dev/sdb1
# vgextend VolGroup00 /dev/sdb1

Now you have free space in your volume group that can be used by logical volumes in this group.

Information: You can add partitions from any disks to volume groups.

Remove partition from a volume group

All of the data on that partition needs to be moved to another partition. Fortunately, LVM makes this easy:

# pvmove /dev/mapper/myvg-mypv

If you want to have the data on a specific physical volume, specify that as the second argument to pvmove.

Then the physical volume needs to be removed from the volume group:

# vgreduce myVg /dev/mapper/myvg-mypv

Or remove all empty physical volumes:

# vgreduce --all vg0

And lastly, if you want to use the partition for something else, and want to avoid lvm thinking that the partition is a physical volume:

# pvremove /dev/mapper/myvg-removedpv

Snapshots

Introduction

LVM allows you to take a snapshot of your system in a much more efficient way than a traditional backup. It does this efficiently by using a COW (copy-on-write) policy. The initial snapshot you take simply contains hard-links to the inodes of your actual data. So long as your data remains unchanged, the snapshot merely contains its inode pointers and not the data itself. Whenever you modify a file or directory that the snapshot points to, LVM automatically clones the data, the old copy referenced by the snapshot, and the new copy referenced by your active system. Thus, you can snapshot a system with 35GB of data using just 2GB of free space so long as you modify less than 2GB (on both the original and snapshot).

Configuration

You create snapshot logical volumes just like normal ones.

# lvcreate --size 100M --snapshot --name snap01 /dev/mapper/vg0-pv

With that volume, you may modify less than 100M of data, before the snapshot volume fills up.

It is important to have the dm-snapshot module listed in the MODULES variable of /etc/mkinitcpio.conf, otherwise the system will not boot. If you do this on an already installed system, make sure to rebuild the image with

# mkinitcpio -g /boot/kernel26.img

Todo: scripts to automate snapshots of root before updates, to rollback... updating menu.lst to boot snapshots (separate article?)

snapshots are primarily used to provide a frozen copy of a filesystem to make backups; a backup taking two hours provides a more consistent image of the filesystem than directly backing up the partition.

Troubleshooting

LVM commands don't work

try preceeding commands with lvm like this:

# lvm pvdisplay

Set Filesystem Mountpoints page doesn't show logical volumes

If you are installing on a system where there is an existing volume group, you may find that even after doing "modprobe dm-mod" you don't see the list of logical volumes.

In this case, you may also need to do:

# vgchange -ay <volgroup>

in order to activate the volume group and make the logical volumes available.

Tips & Tricks

Todo

More Resources

Other LVM articles on the Archwiki:

External resources: