LVM: physical volumes, volume groups and logical volumes

The problem with plain partitions

A partition has a fixed size and lives on one disk. When /data fills up, you cannot just "add another disk to it". LVM adds a layer between disks and file systems that fixes this. It is used on most RHEL servers; the installer itself puts / and swap on LVM by default.

The three layers

A VG is divided into physical extents (PE), small equal blocks. The default PE size is 4 MiB; vgcreate -s 8M chooses another. An LV is always a whole number of extents. Exam tasks sometimes say "make an LV of 20 extents in a VG with 16 MiB extents"; that is 320 MiB.

Building it, step by step

# pvcreate /dev/vdb3 /dev/vdc1
  Physical volume "/dev/vdb3" successfully created.
  Physical volume "/dev/vdc1" successfully created.
# vgcreate -s 8M vgdata /dev/vdb3 /dev/vdc1
  Volume group "vgdata" successfully created
# vgs
  VG     #PV #LV #SN Attr   VSize  VFree
  vgdata   2   0   0 wz--n- <6.98g <6.98g
# lvcreate -n lvweb -L 2G vgdata         # by size
  Logical volume "lvweb" created.
# lvcreate -n lvlogs -l 64 vgdata        # by extents: 64 x 8 MiB = 512 MiB
# lvcreate -n lvrest -l 100%FREE vgdata  # everything left
# lvs
  LV     VG     Attr       LSize   Pool Origin Data%  Meta%
  lvlogs vgdata -wi-a----- 512.00m
  lvrest vgdata -wi-a-----  <4.48g
  lvweb  vgdata -wi-a-----   2.00g

An LV appears as two names that point to the same device: /dev/vgdata/lvweb and /dev/mapper/vgdata-lvweb. Use either in fstab. From here it is like any partition: make a file system, mount it, add it to fstab.

# mkfs.xfs /dev/vgdata/lvweb
# mkdir -p /var/www
# echo "/dev/vgdata/lvweb /var/www xfs defaults 0 0" >> /etc/fstab
# systemctl daemon-reload && mount -a
# lsblk /dev/vdb3
NAME             MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
vdb3             252:19   0    2G  0 part
└─vgdata-lvweb   253:2    0    2G  0 lvm  /var/www

Removing in the reverse order

# umount /var/www          # and remove its fstab line
# lvremove /dev/vgdata/lvweb
# vgremove vgdata
# pvremove /dev/vdb3 /dev/vdc1

Other storage layers (briefly)

RHEL also offers Stratis, a newer tool that manages pools and thin file systems with one command set, and VDO (Virtual Data Optimizer), which saves space by removing duplicate data and compressing it; in RHEL 9 VDO is used through LVM (lvcreate --type vdo). Both are useful in special cases. Learn plain LVM well first; it is what you will meet on almost every server and in the exam. Check the current exam objectives for whether Stratis or VDO are included.

Common mistakes

• Running pvcreate on a disk that has data or a partition table you need. It overwrites the start of the device.
• Mixing up -L (size) and -l (extents): lvcreate -l 2G fails, and lvcreate -L 64 gives 64 MiB, not 64 extents.
• Choosing a PE size the task did not ask for. Some exam tasks specify it; set it with vgcreate -s, because it cannot easily be changed later.
• Forgetting that an LV still needs a file system and a mount.