Exercise: Grow and shrink volumes

Run out of space on purpose, add a disk to the volume group, grow a mounted XFS volume and an ext4 volume while they hold data, and see what happens when you try to shrink each of them.

Grow and shrink volumes

Extend an LV and its file system online with lvextend -r.
Add a physical volume to a volume group.
Shrink ext4 and understand why XFS cannot shrink.

Start the exercise

On workstation, start the exercise. It builds the volume group vgdata on /dev/sdb1 of servera with the volumes lvapp (1 GiB, XFS, /srv/app) and lvlog (ext4, /srv/log), the result of the previous exercise.

[student@workstation ~]$ lab start sa-lvm-resize

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Start from the previous exercise

On servera as root, make sure vgdata with lvapp (xfs, /srv/app) and lvlog (ext4, /srv/log) exist and are mounted. Write a recognisable file into /srv/app.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ sudo -i
[root@servera ~]# vgs; lvs
  VG     #PV #LV #SN Attr   VSize  VFree
  vgdata   1   2   0 wz--n- <1.50g 256.00m
  LV    VG     Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  lvapp vgdata -wi-ao----   1.00g
  lvlog vgdata -wi-ao---- 252.00m
[root@servera ~]# seq 1 100000 > /srv/app/numbers.txt; sha256sum /srv/app/numbers.txt | cut -c1-16
b2bc7d3f8b652d2e

The checksum prefix lets you prove later that the data survived. If you reset the servers, repeat the previous exercise first.

Ask for too much

Try to add 1 GiB to lvapp. Read the message and the free space.

[root@servera ~]# lvextend -L +1G /dev/vgdata/lvapp
  Insufficient free space: 256 extents needed, but only 64 available
[root@servera ~]# vgs
  VG     #PV #LV #SN Attr   VSize  VFree
  vgdata   1   2   0 wz--n- <1.50g 256.00m

Add the second disk

Turn /dev/sdb2 into a PV and add it to vgdata. How much space is free now?

[root@servera ~]# pvcreate /dev/sdb2
  Physical volume "/dev/sdb2" successfully created.
[root@servera ~]# vgextend vgdata /dev/sdb2
  Volume group "vgdata" successfully extended
[root@servera ~]# vgs; pvs
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   2   2   0 wz--n- 2.99g <1.75g
  PV         VG     Fmt  Attr PSize  PFree
  /dev/sdb1  vgdata lvm2 a--  <1.50g 256.00m
  /dev/sdb2  vgdata lvm2 a--  <1.50g  <1.50g

Grow in two steps, then in one

Extend lvapp by 512 MiB without -r and compare lvs with df. Then grow the file system with xfs_growfs. Finally grow lvlog by 256 MiB with -r in a single command.

[root@servera ~]# lvextend -L +512M /dev/vgdata/lvapp
  Size of logical volume vgdata/lvapp changed from 1.00 GiB (256 extents) to 1.50 GiB (384 extents).
  Logical volume vgdata/lvapp successfully resized.
[root@servera ~]# lvs vgdata/lvapp | tail -1; df -h /srv/app | tail -1
  lvapp vgdata -wi-ao---- 1.50g
/dev/mapper/vgdata-lvapp  960M   39M  922M   5% /srv/app
[root@servera ~]# xfs_growfs /srv/app | head -2
meta-data=/dev/mapper/vgdata-lvapp isize=512    agcount=4, agsize=65536 blks
         =                       sectsz=512   attr=2, projid32bit=1
[root@servera ~]# df -h /srv/app | tail -1
/dev/mapper/vgdata-lvapp  1.5G   43M  1.4G   3% /srv/app
[root@servera ~]# lvextend -r -L +256M /dev/vgdata/lvlog | tail -2
  Extended file system ext4 on vgdata/lvlog.
  Logical volume vgdata/lvlog successfully resized.
[root@servera ~]# df -h /srv/log | tail -1
/dev/mapper/vgdata-lvlog  471M   14K  444M   1% /srv/log

After the plain lvextend, lvs showed 1.5 GiB but df still 960 MiB; the file system only grew once told. -r combines the two steps.

Use up the rest

Give all remaining free space to lvapp, with its file system. Check the data.

[root@servera ~]# lvextend -r -l +100%FREE /dev/vgdata/lvapp | tail -2
  Extended file system xfs on vgdata/lvapp.
  Logical volume vgdata/lvapp successfully resized.
[root@servera ~]# vgs; df -h /srv/app | tail -1; sha256sum /srv/app/numbers.txt | cut -c1-16
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   2   2   0 wz--n- 2.99g    0
/dev/mapper/vgdata-lvapp  2.5G   51M  2.4G   2% /srv/app
b2bc7d3f8b652d2e

The group has no free space left, the volume is 2.5 GiB, and the checksum is unchanged: nothing was lost or unmounted.

Shrink ext4, fail to shrink XFS

Unmount /srv/log, shrink lvlog to 200 MiB with lvreduce -r, mount it again, and check the size. Then try to shrink lvapp to 500 MiB.

[root@servera ~]# umount /srv/log
[root@servera ~]# lvreduce -r -L 200M /dev/vgdata/lvlog
resize2fs done
  Reduced file system ext4 on vgdata/lvlog.
  Size of logical volume vgdata/lvlog changed from 508.00 MiB (127 extents) to 200.00 MiB (50 extents).
  Logical volume vgdata/lvlog successfully resized.
[root@servera ~]# mount /dev/vgdata/lvlog /srv/log; df -h /srv/log | tail -1
/dev/mapper/vgdata-lvlog  183M   14K  170M   1% /srv/log
[root@servera ~]# lvreduce -L 500M /dev/vgdata/lvapp
  File system size (<2.50 GiB) is larger than the requested size (500.00 MiB).
  File system reduce is required and not supported (xfs).

ext4 shrank (offline); XFS refused. Your exact sizes differ slightly.

Grade and finish

Check your work with lab grade sa-lvm-resize, then put the exercise away with lab finish sa-lvm-resize.