LVM¶

Physical Volumes (PVs)¶

Creating a Physical Volume¶

pvcreate /dev/mapper/extBackup

Expanding a Physical Volume¶

sudo pvresize /dev/mapper/Array0

Removing a PV¶

sudo pvremove /dev/vdd

Potential Issues¶

Device /dev/X not found (or ignored by filtering)¶

$ sudo pvcreate /dev/sdb
  Device /dev/sdb not found (or ignored by filtering).

# dd if=/dev/zero of=/dev/sdb
^C845369+0 records in
845369+0 records out
432828928 bytes (433 MB, 413 MiB) copied, 9.88539 s, 43.8 MB/s

# pvcreate /dev/sdb
  Physical volume "/dev/sdb" successfully created

Volume Groups (VGs)¶

Creating a Volume Group¶

vgcreate extBackup1 /dev/mapper/extBackup

Removing a VG¶

sudo vgremove raspbian

Renaming a VG¶

sudo vgrename /dev/oldname /dev/newname

Moving a VG to another PV¶

Add new disk (xvdf)

pvcreate /dev/xvdf

vgextend vg0 /dev/xvdf

pvmove /dev/xvdb /dev/xvdf

vgreduce vg0 /dev/xvdb

Aborting¶

pvmove --abort

vgreduce vg0 /dev/xvdf

Logical Volumes (LVs)¶

Creating a Logical Volume¶

lvcreate -L 10737418240b -n JFNZWGPCAT01 512SSD
lvcreate -L 10G -n /dev/512SSD/Swap

Expanding a Logical Volume¶

sudo lvextend --size +2T /dev/Array0_Decrypted/Array0_Data

sudo lvextend -l +100%FREE /dev/Array0_Decrypted/Array0_Data

sudo lvextend -l +6 /dev/mapper/vg0-home [Extend by extents]

Removing an LV¶

sudo lvremove /dev/mapper/ubuntu-whatever-lv

Locate a LV¶

In the below example the Segments show the LV is on /dev/vda. Useful for multi-PV LVs.

# lvdisplay --maps
  --- Logical volume ---
  LV Name                /dev/vg0/srv
  VG Name                vg0
  LV UUID                c9hbjS-ui4Y-iVLw-anQ3-wkWm-0rMd-RVaaWk
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                22.00 GiB
  Current LE             5631
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           251:3

  --- Segments ---
  Logical extent 0 to 5630:
    Type                linear
    Physical volume     /dev/vda
    Physical extents    14848 to 20478

Conversion Tasks¶

Convert QCOW2 to LVM¶

qemu-img convert disk0.qcow2 -O raw disk0.raw

sudo dd if=disk.raw of=/dev/vgroup/lv_disk

SSD Caching¶

In this example: adding multiple caches on an SSD (encrypted via LUKS) to a VG on a slow array (4 x 4TB HDDs) with pre-existing LVs.

The SSD is /dev/mapper/ssd and the VG is vg0.

Create the PV on the SSD¶

pvcreate /dev/mapper/ssd

Extend the original volume group¶

vgextend vg0 /dev/mapper/ssd

Create LV caches¶

lvcreate -L 40G -n cache0 vg0 /dev/mapper/ssd
lvcreate -L 40G -n cache1 vg0 /dev/mapper/ssd
lvcreate -L 40G -n cache2 vg0 /dev/mapper/ssd

Add the cache to the existing logical volumes¶

lvconvert --type cache --cachepool vg0/cache0 vg0/lv0
lvconvert --type cache --cachepool vg0/cache1 vg0/lv1
lvconvert --type cache --cachepool vg0/cache2 vg0/lv2

Check cache utilisation¶

You can see utilization with lvs -a -o +devices.

Writeback caching is not recommended unless your data is easily replaceable.

File System Tasks¶

Resize EXT4¶

sudo resize2fs /dev/Array0_Decrypted/Array0_Media

Resize XFS¶

sudo xfs_growfs /dev/centos/root

Quick getting started guide for people who haven't used LVM before¶

Have or add a disk to the system, use lsblk or similar to get its path (/dev/sdx used in this example)
Create a Physical Volume

pvcreate /dev/sdx

Create a Volume Group

Replace <vgname> with the name you want to use for your Volume Group (vg0, data, etc)

vgcreate <vgname> /dev/sdx

Create a Logical Volume Replace <lvname> with the name you want to use for your Logical Volume. For ease I'd suggest using something like var-lib-influxdb if you plan to mount the LV at /var/lib/influxdb.

Replace <size> with something like 20G or your desired size.

lvcreate -L <size> -n /dev/<vgname>/<lvname>

Create a file system on the LV Using ext4 as an example:

mkfs.ext4 /dev/<vgname>/<lvname>

Create a new directory and mount the LV

If you are intending to replace an existing directory skip to Step 7.

Create the new directory

mkdir /path/to/mount

Update /etc/fstab with the new mount details

/dev/<vgname>/<lvname> /path/to/mount ext4 errors=remount-ro   0       2

Mount it via the mount command to make sure fstab works

mount -a

Mount the LV over an existing directory

Follow this process if you're wanting to replace an existing directory with an LV, normally done if your root disk is low on space and you want to move something like /var/log off.

As part of this process we create a temporary directory, mount the LV to the temporary directory, move the data accross, check the permissions and then unmount.

Before doing this you should make sure you created a LV large enough to hold the data from the directory.

Make a note of the permissions on the original directory

Using the example of /path/to/mount do an ls -la one directory down

ls -la /path/to/

Create a temporary directory

mkdir /path/to/temp-temp-temp

Mount the new LV on the temporary directory

mount /dev/<vgname>/<lvname> /path/to/temp-temp-temp

Stop any services using the directory! If you are moving files that are being actively used, will be automatically replaced or are required by a running process you will have an unhappy system and may lose all the data you're trying to make room for.

Some example services would be docker for /var/lib/docker or rsyslogd in /var/log. This will vary based on operating system, the version of the OS and any changes that have been made to the host since installation.

Move the directory contents

If you're less confident about this and don't mind cleaning the old directory afterwards use rsync instead.

mv /path/to/mount/* /path/to/temp-temp-temp/

Unmount and remove the temporary directory

umount /path/to/temp-temp-temp/ && rm -r /path/to/temp-temp-temp/

Update /etc/fstab with the new mount details

/dev/<vgname>/<lvname> /path/to/mount ext4 errors=remount-ro   0       2

Mount it via the mount command to make sure fstab works

mount -a

Using the output from the ls -la command earlier set the appropriate user, group and permission settings on the mounted directory

For example if you are trying to relpace /var/log/ the ls -la output for /var/ is something like this:

$ ls -la /var
total 56
drwxr-xr-x 14 root root   4096 May 11  2021 .
drwxr-xr-x 23 root root   4096 May  6 01:38 ..
drwxr-xr-x  2 root root   4096 May  8 06:25 backups
drwxr-xr-x 10 root root   4096 May 31  2021 cache
drwxrwxrwt  2 root root   4096 Jan 26  2019 crash
drwxr-xr-x 44 root root   4096 Apr 24 06:13 lib
drwxrwsr-x  2 root staff  4096 Apr 24  2018 local
lrwxrwxrwx  1 root root      9 Jan 26  2019 lock -> /run/lock
drwxrwxr-x 12 root syslog 4096 May  8 06:25 log
drwxrwsr-x  2 root mail   4096 Jan 26  2019 mail
drwxr-xr-x  2 root root   4096 Jan 26  2019 opt
drwxr-xr-x  3 root root   4096 May  6 01:38 osquery
lrwxrwxrwx  1 root root      4 Jan 26  2019 run -> /run
drwxr-xr-x  2 root root   4096 Jul 19  2018 snap
drwxr-xr-x  4 root root   4096 Jan 26  2019 spool
drwxrwxrwt  6 root root   4096 May  8 21:17 tmp

So you would want to make /var/log owned by root, with the group root and with drwxrwxr-x permissions. This will vary with each directory, please don't assume these are the right permissions - that's why I haven't put the commands here.

Start any services you stopped that were using the directory

If they start fine I'd then recommend restarting the system to make sure it survives a reboot.