Build and resize logical volumes (LVM)

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$ als gewone gebruiker# als root (of met sudo)

Examendoelen in dit hoofdstuk

Fysieke volumes maken en verwijderen
Fysieke volumes aan volumegroepen toevoegen
Logische volumes maken en verwijderen
Bestaande logische volumes groter maken

Understand the LVM layers

LVM has three layers: disks become physical volumes (PV), PVs are joined into a volume group (VG), and from the VG you cut logical volumes (LV). The file system goes on the LV.

LVM layers

 file systems:   /data (xfs)        /web (ext4)
 logical vols:   lvdata 5G          lvweb 3G
                 └────────── vgdata (VG, 12G) ──────────┘
 physical vols:  /dev/vdb1 (PV)  /dev/vdc (PV)

Term	Meaning
PV	A disk or partition prepared for LVM
VG	The storage pool made of one or more PVs
LV	A "partition" cut from the VG; it gets a file system
PE / extent	The unit of space in a VG (default 4 MiB)

15.2

Know what LVM can do

The main reasons to use LVM.

Resize: grow (and for ext4 also shrink) volumes, add disks to a VG while it runs.
Join disks: one LV can use space from many disks.
Snapshots: a frozen view of an LV for backups.
Move data: pvmove moves data off a disk while it is in use, so you can replace the disk.
Thin provisioning, RAID and cache LVs (more advanced).

Notitie

RHEL installs the root and swap on LVM by default (VG rhel).

15.3

Create physical volumes

A PV can be a whole disk or a partition. With partitions, set the type to lvm first.

(Partition way) Create a partition with type lvm.

#fdisk /dev/vdb
Command (m for help): n          (+5G)
Command (m for help): t          (lvm)
Command (m for help): w

Turn the partition (or a whole disk) into a PV.

#pvcreate /dev/vdb1
  Physical volume "/dev/vdb1" successfully created.
#pvcreate /dev/vdc

Check the PVs.

#pvs
  PV         VG   Fmt  Attr PSize   PFree
  /dev/vda2  rhel lvm2 a--  <19.00g     0
  /dev/vdb1       lvm2 ---    5.00g  5.00g
  /dev/vdc        lvm2 ---   10.00g 10.00g
#pvdisplay /dev/vdb1

15.4

Create a volume group

Join one or more PVs into a VG. You can choose the extent size with -s (default 4 MiB).

#vgcreate vgdata /dev/vdb1 /dev/vdc
  Volume group "vgdata" successfully created
#vgs
  VG     #PV #LV #SN Attr   VSize   VFree
  vgdata   2   0   0 wz--n- <14.99g <14.99g
#vgdisplay vgdata
#vgcreate -s 8M vgbig /dev/vdd

Examentip

A task may ask for an extent size ("extents of 8 MiB") and an LV size in extents ("20 extents"): use vgcreate -s 8M and lvcreate -l 20. Check with vgdisplay (PE Size).

15.5

Create logical volumes and file systems

lvcreate cuts an LV from the VG: with -L a size, with -l a number of extents or a percentage.

Create the LVs.

#lvcreate -n lvdata -L 5G vgdata
#lvcreate -n lvweb -l 50%FREE vgdata
#lvcreate -n lvlog -l 100 vgdata
#lvs

Put a file system on them and mount them for good.

#mkfs.xfs /dev/vgdata/lvdata
#mkdir /data
#echo "/dev/vgdata/lvdata /data xfs defaults 0 0" >> /etc/fstab
#systemctl daemon-reload; mount -a

lvcreate size	Means
`-L 5G`	5 GiB
`-L 500M`	500 MiB
`-l 100`	100 extents
`-l 50%FREE`	Half of the free space in the VG
`-l 100%FREE`	All free space
`-l 20%VG`	20 % of the whole VG

15.6

Name and find LVM devices

An LV has several device names. All point to the same device-mapper device (/dev/dm-N).

Name	Example
`/dev/VG/LV`	`/dev/vgdata/lvdata`
`/dev/mapper/VG-LV`	`/dev/mapper/vgdata-lvdata`
`/dev/dm-N`	`/dev/dm-3` (can change; do not use it)

$ls -l /dev/vgdata/ /dev/mapper/
$lsblk /dev/vdc
#lvdisplay /dev/vgdata/lvdata

Onthoud

In fstab, /dev/VG/LV or /dev/mapper/VG-LV are stable names; you do not need the UUID for LVs.

15.7

Add space to a volume group

When the VG is full, add a new PV with vgextend.

#pvcreate /dev/vdd
#vgextend vgdata /dev/vdd
#vgs vgdata

15.8

Grow a logical volume and its file system

lvextend -r grows the LV and the file system in one step, while it is mounted.

#lvextend -r -L +2G /dev/vgdata/lvdata
#lvextend -r -l +100%FREE /dev/vgdata/lvweb
#lvextend -r -L 10G /dev/vgdata/lvlog
$df -h /data

	Meaning
`-L +2G`	Add 2 GiB
`-L 10G`	Make it 10 GiB in total
`-l +100%FREE`	Add all free space of the VG
`-r`	Also resize the file system (xfs_growfs or resize2fs)

Without -r, grow the file system yourself.

#lvextend -L +2G /dev/vgdata/lvdata
#xfs_growfs /data
#resize2fs /dev/vgdata/lvweb

Examentip

Forgetting the file system is the classic mistake: the LV is bigger but df -h shows the old size. Always use lvextend -r.

15.9

Shrink volumes and reduce a volume group

XFS cannot shrink. ext4 can, but only unmounted — lvreduce -r does the steps for you. To remove a disk from a VG, first move its data away.

Shrink an ext4 LV (with its file system).
```
#lvreduce -r -L 2G /dev/vgdata/lvweb
```

Move data off a PV and remove it from the VG.

#pvmove /dev/vdc
#vgreduce vgdata /dev/vdc
#pvremove /dev/vdc

Remove an LV and a VG you do not need anymore.

#umount /data
#lvremove /dev/vgdata/lvdata
#vgremove vgdata

Let op

Shrinking an LV without shrinking the file system first destroys the file system. Always use -r (or run resize2fs first). Remove the fstab line before you delete an LV, or the next boot fails.

15.10

Remember the main points

The most important facts of this chapter, in short.

LVM has three layers: physical volumes (PV) → volume group (VG) → logical volumes (LV).
Commands: pvcreate, vgcreate, lvcreate; show with pvs, vgs, lvs (or the display versions).
The VG is split in extents (default 4 MiB); vgcreate -s 8M sets another extent size; lvcreate -l 100 gives 100 extents, -L 500M gives a size.
LVs appear as /dev/VG/LV and /dev/mapper/VG-LV.
Make a VG bigger with vgextend VG /dev/new-pv.
Grow an LV and its file system together: lvextend -r -L +1G /dev/VG/LV (-r runs xfs_growfs or resize2fs).
Only ext4 can shrink: lvreduce -r -L 3G …. Move data off a PV with pvmove, then vgreduce.

15.11

Learn the key terms

The words of this chapter in simple English.

Term	Meaning
LVM	Logical Volume Manager: flexible storage on top of disks.
Physical volume (PV)	A disk or partition prepared for LVM.
Volume group (VG)	A pool of space made from one or more PVs.
Logical volume (LV)	A part of a VG, used like a partition.
Physical extent (PE)	The smallest block of space in a VG.
Device mapper	The kernel part that makes LVs appear as devices.
Snapshot	A frozen copy of an LV at one moment.
Thin provisioning	LVs that can be bigger than the space really used.