virt-resize –shrink now works

(Shrinking is still tedious, but at least it now works. Expanding is much simpler).

Look at the original disk image and decide what scope there is for shrinking it. In this case we could reduce the size by as much as 5.5GB, but for the purpose of this test I will only shrink the image from 10GB to 7GB.

$ virt-df /tmp/disk.img
Filesystem                           1K-blocks       Used  Available  Use%
disk.img:/dev/sda1                      495844      29565     440679    6%
disk.img:/dev/vg_f13x64/lv_root        7804368    2077504    5647596   27%

We can expand guests automatically, but virt-resize is much more conservative about the complex business of shrinking guests. You have to first use guestfish on (a copy of) the original disk to manually shrink the content of the partitions you want to shrink. In this case I manually shrink the root filesystem, its LV, and the PV.

$ guestfish -a /tmp/disk.img

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for a list of commands
      'man' to read the manual
      'quit' to quit the shell

><fs> run
><fs> resize2fs-size /dev/vg_f13x64/lv_root 4G
libguestfs: error: resize2fs_size: resize2fs 1.41.12 (17-May-2010)
Please run 'e2fsck -f /dev/vg_f13x64/lv_root' first.

This known problem with resize2fs is documented.

><fs> e2fsck-f /dev/vg_f13x64/lv_root
><fs> resize2fs-size /dev/vg_f13x64/lv_root 4G
><fs> lvresize /dev/vg_f13x64/lv_root 4096
><fs> pvresize-size /dev/sda2 6G
libguestfs: error: pvresize_size: /dev/vda2:
/dev/vda2: cannot resize to 191 extents as later ones are allocated.

This is unexpected: pvresize does not “defrag”. Perhaps in a future version of pvresize, pvmove or libguestfs we will add this, but for now I have to workaround it by deleting the troublesome swap partition, doing the resize, then recreating the swap.

><fs> lvremove /dev/vg_f13x64/lv_swap
><fs> pvresize-size /dev/sda2 6G
><fs> lvcreate lv_swap vg_f13x64 512
><fs> mkswap /dev/vg_f13x64/lv_swap
><fs> exit

Now I can perform the resize operation itself using virt-resize. Note this is also a copy operation, since virt-resize never changes the source disk.

$ truncate -s 7G /tmp/disk2.img
$ virt-resize --shrink /dev/sda2 /tmp/disk.img /tmp/disk2.img
Summary of changes:
/dev/sda1: partition will be left alone
/dev/sda2: partition will be resized from 9.5G to 6.5G
Copying /dev/sda1 ...
[########################################################]
Copying /dev/sda2 ...
[########################################################]

After a test boot, the final guest worked(!)

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