LVM is the implementation of logical volume management in Linux. As I don’t use it on a day-to-day basis, I created this blog in case I forgot the basics :-).

Terminology

       sda1   sdc       (PVs on partitions or whole disks)
          \   /
           \ /
          diskvg        (VG)
          /  |  \
         /   |   \
     usrlv rootlv varlv (LVs)
       |      |     |
    ext4  reiserfs  xfs (filesystems)

Physical volume (PV) – partition (ex. /dev/sda1), disk (ex. /dev/sdc) or RAID device (ex. /dev/md0)
Volume group (VG) – group of physical volumes (ex. diskvg)
Logical volume (LV) – equivalent of standard partitions, where filesystems can be created (ex. /usrlv)

Working with LVM

Creating Volumes

Create PV (initialize disk)
```
pvcreate /dev/md0
```
Check the results with pvdisplay
Create VG
```
vgcreate rootvg /dev/md0
```
Check the results with vgdisplay

Create LV

lvcreate --name backuplv --size 50G rootvg

Check the results with lvdisplay

Create filesystem
```
mkfs.ext3 /dev/rootvg/backuplv
```

Edit /etc/fstab

# RAID 1 + LVM
/dev/rootvg/backuplv   /backup        ext3    rw,noatime      0       0

Create mount point and mount volume(s)
```
mkdir /backup
mount /backup
```

Extending LV

Extend the LV
```
lvextend -L +5G /dev/rootvg/backuplv
```
Re-size the filesystem (online re-sizing doesn’t seem to cause troubles)
```
resize2fs /dev/rootvg/backuplv
```

when shrinking, first resize filesystem then shrink the LV.

Snapshotting LV (e.g. for doing backups)

lvcreate -L 50G -s -n backuplv-snap rootvg/backuplv  # should be short-lived or of the same size as source LV

in theory /backup should by first unmounted to ensure consistency, in practice etx4 protects us aginst filesystem corruption although we may lose a few of recent data blocks updates (perfectly OK for backup purposes)