LVM cheat sheet

Example commands for common LVM related tasks.

RAID

Convert existing LV to RAID1

Extend VG containing the LV with additional PV before conversion.

BASH

# vgextend vgdev /dev/nvme0n1p2
# lvconvert --type raid1 --mirrors 1 vgdev/backup /dev/nvme0n1p2

List currently used devices and sync status:

BASH

# lvs -a -o +devices

Fix RAID inconsistency due to hardware errors

Run scrubbing – a full scan of RAID LV, which verifies RAID metadata, LV data and parity blocks:

BASH

# lvchange --syncaction check /dev/vgdev/backup

If problems are found, this is denoted by m on 9th bit of attributes displayed by lvs:

BASH

$ cat /tmp/lvs
LV      VG    Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync  Convert
backup  vgdev rwi-aor-m- 4.00g                                     100.00

In such case repair ir required. As of LVM tools 2.03.21(2) (2023-04-21) lvchange --syncaction repair is unable to deduce which data is correct. The only safe choice is to rebuild PV on the failed device:

BASH

# lvchange --rebuild /dev/sdb1 /dev/vgdev/backup

Rebuild RAID1 LV with integrity layer

As of LVM tools 2.03.21(2) (2023-04-21) man lvmraid says:

Integrity limitations

[…] The following are not yet permitted on RAID LVs with integrity: lvreduce, pvmove, snapshots, splitmirror, raid syncaction commands, raid rebuild.

That means commands like lvchange --syncaction repair or lvchange --rebuild won’t work as expected.

If RAID1 with integrity LV requires refresh (partial synchronization) due to e.g. device write errors, but both PVs are otherwise available, take the following steps to fix it:

remove integrity layer:

BASH

# lvconvert --raidintegrity n vgdev/backup

if it is known which PV failed, rebuild it:
BASH
```
# lvchange --rebuild /dev/sda vgdev/backup
```
and re-add integrity layer to finish. Otherwise split RAID1 into single LVs. lvchange --synaction repair could theoretically refresh RAID1. But due to lack of integrity layer, it cannot tell which drive contains uncorrupted data, so the process must be performed manually:
BASH
```
# lvconvert --splitmirrors 1 --name backup_split vgdev/backup
```

check integrity of both RAID1 parts:

whether they differ at all:

BASH

# sha512sum /dev/vgdev/backup /dev/vgdev/backup_split

if LVs differ, check filesystems:

BASH

# fsck.ext4 -n -f /dev/vgdev/backup
# fsck.ext4 -n -f /dev/vgdev/backup_split

once filesystems are fixed and can be mounted, compare file content to determine which mirror is valid:

BASH

# mount /dev/vgdev/backup /mnt/mirror1
# mount /dev/vgdev/backup_split /mnt/mirror1
# rsync -d /mnt/mirror1 /mnt/mirror2

finally remove invalid mirror, recreate RAID1 and re-add integrity layer