Logical Volume Manager (LVM)

Dec 4, 2018

“LVM stands for Logical Volume Management. It is a system of managing logical volumes, or filesystems, that is much more advanced and flexible than the traditional method of partitioning a disk into one or more segments and formatting that partition with a filesystem."

The only way to have a LVM file system on your entire system is a hard format. While installing Ubuntu Server, select partitioning method as “Guided - use entire disk and set up LVM”: https://tutorials.ubuntu.com/tutorial/tutorial-install-ubuntu-server-1604#7

(Tested on ubuntu:16.04 and ubuntu:18.04)

References:

• https://www.digitalocean.com/community/tutorials/how-to-use-lvm-to-manage-storage-devices-on-ubuntu-16-04
• https://wiki.ubuntu.com/Lvm
• https://help.ubuntu.com/lts/serverguide/advanced-installation.html.en#lvm
• http://tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html

You can check your filesystem with:

$ df -h
Filesystem                         Size  Used Avail Use% Mounted on
udev                                16G     0   16G   0% /dev
tmpfs                              3.2G  1.7M  3.2G   1% /run
/dev/mapper/ubuntu--vg-ubuntu--lv 1008G   34G  934G   4% /
tmpfs                               16G     0   16G   0% /dev/shm
tmpfs                              5.0M     0  5.0M   0% /run/lock
tmpfs                               16G     0   16G   0% /sys/fs/cgroup
/dev/loop0                          89M   89M     0 100% /snap/core/5897
/dev/loop1                          87M   87M     0 100% /snap/core/4917
/dev/loop2                          88M   88M     0 100% /snap/core/5742
/dev/sdb2                          976M  144M  766M  16% /boot
tmpfs                              3.2G     0  3.2G   0% /run/user/1000

As seen the filesystem /dev/mapper/ubuntu--vg-ubuntu--lv which is mounted on /, we have our LVM ready.

Scan your LVM setup

There are 3 concepts that LVM manages:

• Physical Volumes
• Volume Groups
• Logical Volumes

You can scan your existing physical volumes with

sudo pvs

Similarly, scan volume groups with vgs and logical volumes with lvs.

Create or Extend a Volume Group

Upon partitioning with LVM, you would end up with a volume group and a logical volume. Physical volumes are associated with underlying hard drives to be used in volume groups. Check disks with:

$ sudo lsblk
NAME                      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0                       7:0    0  88.2M  1 loop /snap/core/5897
loop1                       7:1    0  86.9M  1 loop /snap/core/4917
loop2                       7:2    0  87.9M  1 loop /snap/core/5742
sda                         8:0    0 465.8G  0 disk
├─ubuntu--vg-ubuntu--lv   253:0    0     1T  0 lvm  /
└─ubuntu--vg-drbd--lv     253:2    0 448.1G  0 lvm
  └─drbd0                 147:0    0   448G  0 disk
sdb                         8:16   0 111.8G  0 disk
├─sdb1                      8:17   0     1M  0 part
├─sdb2                      8:18   0     1G  0 part /boot
└─sdb3                      8:19   0 110.8G  0 part
  └─ubuntu--vg-ubuntu--lv 253:0    0     1T  0 lvm  /
sdc                         8:32   0 931.5G  0 disk
├─ubuntu--vg-ubuntu--lv   253:0    0     1T  0 lvm  /
└─ubuntu--vg-swap_1       253:1    0    36G  0 lvm

Here we have sda, sdb, sdc devices. To add a device as physical volume, run:

sudo pvcreate /dev/sda

Once physical volume is created, you can create a volume group with:

sudo vgcreate ubuntu-vg /dev/sda

or expand an existing volume group with:

sudo vgextend ubuntu-vg /dev/sda
sudo vgextend ubuntu-vg /dev/sdb /dev/sdc

You can add as many physical volumes as you like to your volume group. Generally, one volume group per server is sufficient.

Create or Resize Logical Volume

Logical volumes will be partitions of a volume group. Logical volumes can be extended or downsized within a volume group. To create a logical volume with a 10G size:

sudo lvcreate -L 10G --name ubuntu-lv ubuntu-vg

or using all the free space remaining on the volume group (You should leave enough free space on volume group though if you are planning to use snapshots, see snapshots section for more information):

sudo lvcreate -l 100%FREE --name ubuntu-lv ubuntu-vg

To grow the size of a logical volume by 5G:

sudo lvresize -L +5G ubuntu-vg/ubuntu-lv

To apply the changes, you must also expand the filesystem underneath the logical volume. To handle the filesystem expansion for ext4, run:

sudo resize2fs /dev/ubuntu-vg/ubuntu-lv

Reduce the size of Logical Volume

To reduce the size, you must unmount the filesystem. By booting with a live installation USB medium, you can unmount it (it may already not be mounted):

sudo umount /dev/ubuntu-vg/ubuntu-lv

Check the filesystem to ensure that everything is okay:

sudo fsck -t ext4 -f /dev/ubuntu-vg/ubuntu-lv

To resize the ext4 filesystem with the new size of 15G:

sudo resize2fs -p /dev/ubuntu-vg/ubuntu-lv 15G

And resize the logical volume with the same size of 15G:

sudo lvresize -L 15G /dev/ubuntu-vg/ubuntu-lv

Make sure checking the filesystem again with fsck command above. If all are fine, you can mount your filesystem, or simply reboot.

Remove a Physical Volume

If you have enough free space, you can remove a hard drive sda without losing any data. This command will move all the data on sda to other devices so that it is free and can be safely removed.

sudo pvmove /dev/sda

After all the files are transferred, you can exclude the device from your volume group:

sudo vgreduce ubuntu-vg /dev/sda

And finally remove the physical volume associated with sda:

sudo pvremove /dev/sda

You can now remove the disk from your system entirely.

Create snapshots

Snapshots are just another type of logical volumes, so volume group has to provide free space to create a snapshot. But creating snapshot of a 500G volume doesn’t mean you need another 500G of free space. “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." At initial construction, snapshots don’t hold any space. As the original volume changes, those records are reversed and stored in the snapshot to generate a previous state from the current state of the original volume. If the snapshot logical volume is out of space, it will become unusable and be dropped.

To create a snapshot volume with a size of 100G named backup_snapshot of the logical volume ubuntu-lv:

sudo lvcreate -L 100G -s -n backup_snapshot /dev/ubuntu-vg/ubuntu-lv

You can now create a mount point and mount your snapshot volume, if you would like to make file operations on it, i.e. taking a backup.

sudo mkdir -p /mnt/backup_snapshot
sudo mount /dev/ubuntu-vg/backup_snapshot /mnt/backup_snapshot

“You should remove snapshot volume when you have finished with them because they take a copy of all data written to the original volume and this can hurt performance." To unmount and remove the snapshot:

sudo umount /mnt/backup_snapshot
sudo lvremove /dev/ubuntu-vg/backup_snapshot

This way, you can take backup of a live system without taking the service down or having to worry about file modifications during the backup.