From Wikipedia:Logical Volume Manager (Linux):

LVM is a logical volume manager for the Linux kernel; it manages disk drives and similar mass-storage devices.

LVM Building Blocks

Logical Volume Management utilizes the kernel’s device-mapper feature to provide a system of partitions independent of underlying disk layout. With LVM you abstract your storage and have “virtual partitions”, making extending/shrinking easier (subject to potential filesystem limitations).

Virtual partitions allow addition and removal without worry of whether you have enough contiguous space on a particular disk, getting caught up fdisking a disk in use (and wondering whether the kernel is using the old or new partition table), or, having to move other partitions out of the way. This is strictly an ease-of-management solution: LVM adds no security.

Basic building blocks of LVM:

• Physical volume (PV): Partition on hard disk (or even the 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 used to build your hard drive.
• Volume group (VG): Group of physical volumes used as a 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): The smallest size in the physical volume that can be assigned to a logical volume (default 4MiB). Think of physical extents as parts of disks that can be allocated to any partition.

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             |
    |_ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |

Advantages

LVM gives you more flexibility than just using normal hard drive partitions:

• Use any number of disks as one big disk.
• Have logical volumes stretched over several disks.
• Create small logical volumes and resize them “dynamically” as they get filled up.
• Resize logical volumes regardless of their order on disk. It does not depend on the position of the LV within VG, there is no need to ensure surrounding available space.
• Resize/create/delete logical and physical volumes online. File systems on them still need to be resized, but some (such as ext4) support online resizing.
• Online/live migration of LV being used by services to different disks without having to restart services.
• Snapshots allow you to backup a frozen copy of the file system, while keeping service downtime to a minimum.
• Support for various device-mapper targets, including transparent filesystem encryption and caching of frequently used data.

Disadvantages

• Additional steps in setting up the system, more complicated.

Installing Arch Linux on LVM

You should create your LVM Volumes between the partitioning and formatting steps of the installation procedure. Instead of directly formatting a partition to be your root file system, the file system will be created inside a logical volume (LV).

Make sure the lvm2 package is installed.

Quick overview:

• Create partition(s) where your PV(s) will reside. Set the partition type to ‘Linux LVM’, which is 8e if you use MBR, 8e00 for GPT.
• Create your physical volumes (PVs). If you have one disk it is best to just create one PV in one large partition. If you have multiple disks you can create partitions on each of them and create a PV on each partition.
• Create your volume group (VG) and add all PVs to it.
• Create logical volumes (LVs) inside that VG.
• Continue with “Format the partitions” step of Beginners’ guide.
• When you reach the “Create initial ramdisk environment” step in the Beginners Guide, add the lvm hook to /etc/mkinitcpio.conf (see below for details).

Create partitions

If LVM has to be set on the entire disk, there is no need to create any partitions.

Otherwise, partition the device as required before configuring LVM.

Create physical volumes

To list all your devices capable of being used as a physical volume:

# lvmdiskscan

Create a physical volume on them:

# pvcreate DEVICE

This command creates a header on each device so it can be used for LVM. As defined in #LVM Building Blocks, DEVICE can be a disk (e.g. /dev/sda), a partition (e.g. /dev/sda2) or a loop back device. For example:

# pvcreate /dev/sda2

You can track created physical volumes with:

# pvdisplay

Create volume group

The next step is to create a volume group on this physical volume.

First you need to create a volume group on one of the physical volumes:

# vgcreate <volume_group> <physical_volume>

For example:

# vgcreate VolGroup00 /dev/sda2

Then add to it all other physical volumes you want to have in it:

# vgextend <volume_group> <physical_volume>
# vgextend <volume_group> <another_physical_volume>
# ...

For example:

# vgextend VolGroup00 /dev/sdb1
# vgextend VolGroup00 /dev/sdc

You can track how your volume group grows with:

# vgdisplay

Create in one step

LVM allows you to combine the creation of a volume group and the physical volumes in one easy step. For example, to create the group VolGroup00 with the three devices mentioned above, you can run:

# vgcreate VolGroup00 /dev/sda2 /dev/sdb1 /dev/sdc

This command will first set up the three partitions as physical volumes (if necessary) and then create the volume group with the three volumes. The command will warn you it detects an existing filesystem on any of the devices.

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 <size> <volume_group> -n <logical_volume>

For example:

# lvcreate -L 10G VolGroup00 -n lvolhome

This will create a logical volume that you can access later with /dev/mapper/Volgroup00-lvolhome or /dev/VolGroup00/lvolhome. Same as with the volume groups, you can use any name you want for your logical volume when creating it.

You can also specify one or more physical volumes to restrict where LVM allocates the data. For example, you may wish to create a logical volume for the root filesystem on your small SSD, and your home volume on a slower mechanical drive. Simply add the physical volume devices to the command line, for example:

# lvcreate -L 10G VolGroup00 -n lvolhome /dev/sdc1

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

# lvcreate -l 100%FREE  <volume_group> -n <logical_volume>

You can track created logical volumes with:

# lvdisplay

Create file systems and mount logical volumes

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

# modprobe dm_mod
# vgscan
# vgchange -ay

Now you can create file systems on logical volumes and mount them as normal partitions (if you are installing Arch linux, refer to mounting the partitions for additional details):

# mkfs.<fstype> /dev/mapper/<volume_group>-<logical_volume>
# mount /dev/mapper/<volume_group>-<logical_volume> /<mountpoint>

For example:

# mkfs.ext4 /dev/mapper/VolGroup00-lvolhome
# mount /dev/mapper/VolGroup00-lvolhome /home

Add lvm2 hook to mkinitcpio.conf for root on LVM

In case your root filesystem is on LVM, you will need to make sure the udev and lvm2 mkinitcpio hooks are enabled.

udev is there by default. Edit the file and insert lvm2 between block and filesystems like so:

/etc/mkinitcpio.conf

HOOKS="base udev ... block lvm2 filesystems"

Afterwards, you can continue in normal installation instructions with the create an initial ramdisk step.

Special preparations for root on thinly-provisioned volume

If your root device is on a thinly-provisioned LVM volume (as may be needed by snapper if you don’t trust btrfs), then some more preparations are needed.

First, mkinitcpio by default does not include modules and binaries needed for thin provisioning. Adjust the configuration file to compensate:

/etc/mkinitcpio.conf

MODULES="... dm-thin-pool ..."
BINARIES="/usr/bin/thin_check /usr/bin/pdata_tools ..."

LVM only calls thin_check while booting, but if the check fails, you will need other commands provided by pdata_tools for recovery.

Second, with a large number of snapshots, thin_check runs for a long enough time so that waiting for the root device times out. To compensate, add the rootdelay=60 kernel boot parameter to your boot loader configuration.

Kernel options

If the root file system resides in a logical volume, the root= kernel parameter must be pointed to the mapped device, e.g /dev/mapper/vg-name-lv-name.

You may also need dolvm.

Volume operations

Advanced options

If you need monitoring (needed for snapshots) you can enable lvmetad. For this set use_lvmetad = 1 in /etc/lvm/lvm.conf. This is the default by now.

You can restrict the volumes that are activated automatically by setting the auto_activation_volume_list in /etc/lvm/lvm.conf. If in doubt, leave this option commented out.

Resizing volumes

Physical volumes

After extending or prior to reducing the size of a device that has a physical volume on it, you need to grow or shrink the PV using pvresize.

Growing

To expand the PV on /dev/sda1 after enlarging the partition, run:

# pvresize /dev/sda1

This will automatically detect the new size of the device and extend the PV to its maximum.

Shrinking

To shrink a physical volume prior to reducing its underlying device, add the --setphysicalvolumesize size parameters to the command, e.g.:

# pvresize --setphysicalvolumesize 40G /dev/sda1

The above command may leave you with this error:

 /dev/sda1: cannot resize to 25599 extents as later ones are allocated.
 0 physical volume(s) resized / 1 physical volume(s) not resized

Indeed pvresize will refuse to shrink a PV if it has allocated extents after where its new end would be. One needs to run pvmove beforehand to relocate these elsewhere in the volume group if there is sufficient free space.

Move physical extents

Before moving free extents to the end of the volume, one must run # pvdisplay -v -m to see physical segments. In the below example, there is one physical volume on /dev/sdd1, one volume group vg1 and one logical volume backup.

# pvdisplay -v -m

    Finding all volume groups.
    Using physical volume(s) on command line.
  --- Physical volume ---
  PV Name               /dev/sdd1
  VG Name               vg1
  PV Size               1.52 TiB / not usable 1.97 MiB
  Allocatable           yes 
  PE Size               4.00 MiB
  Total PE              399669
  Free PE               153600
  Allocated PE          246069
  PV UUID               MR9J0X-zQB4-wi3k-EnaV-5ksf-hN1P-Jkm5mW
   
  --- Physical Segments ---
  Physical extent 0 to 153600:
    FREE
  Physical extent 153601 to 307199:
    Logical volume	/dev/vg1/backup
    Logical extents	1 to 153599
  Physical extent 307200 to 307200:
    FREE
  Physical extent 307201 to 399668:
    Logical volume	/dev/vg1/backup
    Logical extents	153601 to 246068

One can observe FREE space are split across the volume. To shrink the physical volume, we must first move all used segments to the beginning.

Here, the first free segment is from 0 to 153600 and leaves us with 153601 free extents. We can now move this segment number from the last physical extent to the first extent. The command will thus be:

# pvmove --alloc anywhere /dev/sdd1:307201-399668 /dev/sdd1:0-92466

/dev/sdd1: Moved: 0.1 %
/dev/sdd1: Moved: 0.2 %
...
/dev/sdd1: Moved: 99.9 %
/dev/sdd1: Moved: 100,0%

Resize physical volume

Once all your free physical segments are on the last physical extent, run # vgdisplay and see your free PE.

Then you can now run again the command:

# pvresize --setphysicalvolumesize size PhysicalVolume

See the result:

# pvs

  PV         VG   Fmt  Attr PSize    PFree 
  /dev/sdd1  vg1  lvm2 a--     1t     500g

Resize partition

Last, you need to shrink the partition with your favorite partitioning tool.

Logical volumes

Growing or shrinking with lvresize

Extend logical volume lv1 within volume group vg1 by 2GB without touching its file system:

# lvresize -L +2G vg1/lv1

Reduce vg1/lv1 of 500MB without resizing its file system (make sure it is already shrunk in that case):

# lvresize -L -500M vg1/lv1

Set vg1/lv1 to 15GB and resize its file sytem all at once:

# lvresize -L 15G -r vg1/lv1

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

# lvresize -l +100%FREE vg/lv

See man lvresize for more detailed options.

Resizing the file system separately

If not using the -r, --resizefs option to lv{resize,extend,reduce} or using a file system unspported by fsadm(8) (Btrfs, ZFS…), you need to manually resize the FS before shrinking the LV or after expanding it.

For example with and ext2/ext3/ext4 file system:

# resize2fs vg/lv

will expand the FS to the maximum size of the underlying LV, while

# resize2fs -M vg/lv

will shrink it to its minimum size. To resize it to a specified size, use:

# resize2fs vg/lv NewSize

Remove logical volume

First, find out the name of the logical volume you want to remove. You can get a list of all logical volumes with:

# lvs

Next, look up the mountpoint of the chosen logical volume:

$ lsblk

Then unmount the filesystem on the logical volume:

# umount /<mountpoint>

Finally, remove the logical volume:

# lvremove <volume_group>/<logical_volume>

For example:

# lvremove VolGroup00/lvolhome

Confirm by typing in y.

Update /etc/fstab as necessary.

You can verify the removal of the logical volume by typing lvs as root again (see first step of this section).

Add physical volume to a volume group

You first create a new physical volume on the block device you wish to use, then extend your volume group

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

This of course will increase the total number of physical extents on your volume group, which can be allocated by logical volumes as you see fit.

Remove partition from a volume group

If you created a logical volume on the partition, remove it first.

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

# pvmove /dev/sdb1

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

# pvmove /dev/sdb1 /dev/sdf1

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

# vgreduce myVg /dev/sdb1

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/sdb1

Deactivate volume group

Just invoke

# vgchange -a n my_volume_group

This will deactivate the volume group and allow you to unmount the container it is stored in.

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.

Reverting the modified ‘pv’ logical volume to the state when the ‘snap01’ snapshot was taken can be done with

# lvconvert --merge /dev/vg0/snap01

In case the origin logical volume is active, merging will occur on the next reboot.(Merging can be done even from a LiveCD)

The snapshot will no longer exist after merging.

Also multiple snapshots can be taken and each one can be merged with the origin logical volume at will.

The snapshot can be mounted and backed up with dd or tar. The size of the backup file done with dd will be the size of the files residing on the snapshot volume. To restore just create a snapshot, mount it, and write or extract the backup to it. And then merge it with the origin.

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 -p linux

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 file system to make backups; a backup taking two hours provides a more consistent image of the file system than directly backing up the partition.

See Create root filesystem snapshots with LVM for automating the creation of clean root file system snapshots during system startup for backup and rollback.

Dm-crypt/Encrypting an entire system#LVM on LUKS and Dm-crypt/Encrypting an entire system#LUKS on LVM.

If you have LVM volumes not activated via the initramfs, enable the lvm-monitoring service, which is provided by the lvm2 package.

LVM Cache (lvmcache)

From man:

The cache logical volume type uses a small and fast LV to improve the performance of a large and slow LV. It does this by storing the frequently used blocks on the faster LV. LVM refers to the small fast LV as a cache pool LV. The large slow LV is called the origin LV. Due to requirements from dm-cache (the kernel driver), LVM further splits the cache pool LV into two devices – the cache data LV and cache metadata LV. The cache data LV is where copies of data blocks are kept from the origin LV to increase speed. The cache metadata LV holds the accounting information that specifies where data blocks are stored (e.g. on the origin LV or on the cache data LV). Users should be familiar with these LVs if they wish to create the best and most robust cached logical volumes. All of these associated LVs must be in the same VG.

Create

The fast method is creating a PV (if necessary) on the fast disk and add it to the existing volume group:

# vgextend dataVG /dev/sdx

Create a cache pool with automatic meta data on sdb, and convert the existing logical volume (dataLV) to a cached volume, all in one step:

# lvcreate --type cache -L 19.9G -n dataLV_cachepool dataVG/dataLV /dev/sdx

Remove

If you ever need to undo the one step creation operation above:

# lvconvert --uncache dataVG/dataLV

This commits any pending writes still in the cache back to the origin LV, then deletes the cache. Other options are available and described in man.

Root device on a cached LV

If your root device is on a cached LV, you must prepare mkinitcpio to include required modules:

/etc/mkinitcpio.conf

MODULES="... dm-cache dm_cache_mq dm_cache_smq ..."

dm_cache_smq is only if you run a linux kernel superior or equal to 4.2

If you fail to do so, you’ll end up in a rescue shell early in the boot process. A way to boot again is to remove the cache as described above (lvconvert may not be available from the rescue shell, but is accessible from the lvm tool). Before leaving the rescue shell, activate the LV.

Graphical Configuration

There is no “official” GUI tool for managing LVM volumes, but system-config-lvm^AUR covers most of the common operations, and provides simple visualizations of volume state. It can automatically resize many file systems when resizing logical volumes.

system-config-lvm requires root access, so the .desktop launcher will not work, and you must launch it with sudo. It does not support thin provisioning pools, and will crash on start-up if you have any.

Troubleshooting

Changes that could be required due to changes in the Arch-Linux defaults

The use_lvmetad = 1 must be set in /etc/lvm/lvm.conf. This is the default now – if you have a lvm.conf.pacnew file, you must merge this change.

LVM commands do not work

• Load proper module:

# modprobe dm_mod

The dm_mod module should be automatically loaded. In case it does not, you can try:

/etc/mkinitcpio.conf:

MODULES="dm_mod ..."

You will need to rebuild the initramfs to commit any changes you made.

• Try preceding commands with lvm like this:

# lvm pvdisplay

Logical Volumes do not show up

If you are trying to mount existing logical volumes, but they do not show up in lvscan, you can use the following commands to activate them:

# vgscan
# vgchange -ay

LVM on removable media

Symptoms:

# vgscan
 Reading all physical volumes.  This may take a while...
 /dev/backupdrive1/backup: read failed after 0 of 4096 at 319836585984: Input/output error
 /dev/backupdrive1/backup: read failed after 0 of 4096 at 319836643328: Input/output error
 /dev/backupdrive1/backup: read failed after 0 of 4096 at 0: Input/output error
 /dev/backupdrive1/backup: read failed after 0 of 4096 at 4096: Input/output error
 Found volume group "backupdrive1" using metadata type lvm2
 Found volume group "networkdrive" using metadata type lvm2

Cause:

Removing an external LVM drive without deactivating the volume group(s) first. Before you disconnect, make sure to:

# vgchange -an volume group name

Fix: assuming you already tried to activate the volume group with # vgchange -ay vg, and are receiving the Input/output errors:

# vgchange -an volume group name

Unplug the external drive and wait a few minutes:

# vgscan
# vgchange -ay volume group name

Resizing a contiguous logical volume fails

If trying to extend a logical volume errors with:

" Insufficient suitable contiguous allocatable extents for logical volume "

The reason is that the logical volume was created with an explicit contiguous allocation policy (options -C y or --alloc contiguous) and no further adjacent contiguous extents are available (see also reference).

To fix this, prior to extending the logical volume, change its allocation policy with lvchange --alloc inherit <logical_volume>. If you need to keep the contiguous allocation policy, an alternative approach is to move the volume to a disk area with sufficient free extents (see [1]).

Command “grub-mkconfig” reports “unknown filesystem” errors

Make sure to remove snapshot volumes before generating grub.cfg.

See also

• LVM2 Resource Page on SourceWare.org
• LVM HOWTO article at The Linux Documentation project
• LVM article at Gentoo wiki
• LVM2 Mirrors vs. MD Raid 1 post by Josh Bryan
• Ubuntu LVM Guide Part 1Part 2 detals snapshots