- LVM is a logical volume manager for the Linux kernel; it manages disk drives and similar mass-storage devices.
- 1 LVM Building Blocks
- 2 Advantages
- 3 Disadvantages
- 4 Installing Arch Linux on LVM
- 5 Configuration
- 6 Troubleshooting
- 7 See also
LVM Building Blocks
Logical Volume Management makes use of the device-mapper feature of the Linux kernel to provide a system of partitions that is independent of the underlying disk's layout. With LVM you can abstract your storage space and have "virtual partitions" which makes it easier to extend and shrink partitions (subject to the filesystem you use allowing this) and add/remove partitions without worrying about whether you have enough contiguous space on a particular disk, without getting caught up in the problems of fdisking a disk that is in use (and wondering whether the kernel is using the old or new partition table) and without having to move other partition out of the way. This is strictly an ease-of-management issue: it does not provide any additional security. However, it sits nicely with the other two technologies we are using.
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 4MiB) that can be assigned to a logical Volume. Think of physical extents as parts of disks that can be allocated to any partition.
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 | |_ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
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 more filled.
- 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. Filesystems on them still need to be resized, but some 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 filesystem, while keeping service downtime to a minimum.
These can be very helpful in a server situation, desktop less so, but you must decide if the features are worth the abstraction.
- Linux exclusive (almost). There is no official support in most other OS (FreeBSD, Windows..).
- Additional steps in setting up the system, more complicated.
- If you use the btrfs filesystem, its Subvolume feature will also give you the benefit of having a flexible layout. In that case, using the additional Abstraction layer of LVM may be unnecessary.
Installing Arch Linux on LVM
You should create your LVM Volumes between the Partitioning and formatting steps of the Installation Procedure. Instead of directly formating a partition to be your root filesystem, it will be created inside a logical volume (LV).
Make sure the installed.package is
- Create partition(s) where your PV will reside. Set the partition type to 'Linux LVM', which is 8e if you use MBR, 8e00 for GPT.
- Create your physical volumes (PV). 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 the PV to it.
- Create logical volumes (LV) inside your 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 mkinitcpio.conf (see below for details).
/bootcannot reside in LVM when using GRUB Legacy, which does not support LVM. GRUB users do not have this limitation. If you need to use GRUB Legacy, you must create a separate /boot partition and format it directly.
Create physical volumes
Make sure you target the right partitions! To find the partitions with type 'Linux LVM':
- MBR system:
- GPT system:
gdisk -l disk-device
Create a physical volume on them:
# pvcreate disk-device
disk-device may be e.g. /dev/sda2. This command creates a header on each partition so it can be used for LVM. You can track created physical volumes with:
pvcreate --dataalignment 1m /dev/sda2(for erase block size < 1MiB), see e.g. here
Create volume group
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:
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
/dev/VolGroup00/lvolhome. 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
-C y is used to create a contiguous partition, which means that your swap space does not 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:
Create filesystems and mount logical volumes
Your logical volumes should now be located in
/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 filesystems on logical volumes and mount them as normal partitions (if you are installing Arch linux, refer to mounting the partitions for additional details):
# mkfs.ext4 /dev/mapper/VolGroup00-lvolhome # mount /dev/mapper/VolGroup00-lvolhome /home
/dev/mapper/Volgroup00-lvolhome). Do not select the actual partitions on which logical volumes were created (do not use:
Add lvm hook to mkinitcpio.conf
You'll need to make sure the
lvm2 mkinitcpio hooks are enabled.
udev is there by default. Edit the file and insert
filesystem like so:
HOOKS="base udev ... block lvm2 filesystems"
Afterwards, you can continue in normal installation instructions with the create an initial ramdisk step.
If you need monitoring (needed for snapshots) you can enable lvmetad.
For this set
use_lvmetad = 1 in
This is the default by now.
You can restrict the volumes that are activated automatically by setting the
/etc/lvm/lvm.conf. If in doubt, leave this option commented out.
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 us 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
lvresize instead of
If you want to fill all the free space on a volume group, use the next command:
# lvextend -l +100%FREE VolGroup00/lvolhome
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 us 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
Here we shrunk the filesystem more than needed so that when we shrunk the logical volume we did not accidentally cut off 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
# lvresize -L -5G VolGroup00/lvolhome # resize2fs /dev/VolGroup00/lvolhome
- Do not reduce the filesystem size to less than the amount of space occupied by data or you risk data loss.
- Not all filesystems support shrinking without loss of data and/or shrinking online.
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 installed on the system with:
Next, look up the mountpoint for your chosen logical volume...:
$ df -h
... and unmount it:
# umount /your_mountpoint
Finally, remove the logical volume:
# lvremove /dev/yourVG/yourLV
Confirm by typing
y and you are done.
Do not forget, to update
You can verify the removal of your 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.
8efor MBR, and
8e00for GPT partitions.
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/sdb1
If you want to have the data on a specific physical volume, specify that as the second argument to
# 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
# vgchange -a n my_volume_group
This will deactivate the volume group and allow you to unmount the container it is stored in.
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).
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 -g /boot/initramfs-linux.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.
See Create root filesystem snapshots with LVM for automating the creation of clean root filesystem snapshots during system startup for backup and rollback.
Changes that could be required due to changes in the Arch-Linux defaults
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
dm_mod module should be automatically loaded. In case it does not, you can try:
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
# 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
- 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
In kernel options, you may need
root= should be set to the logical volume, e.g