Qemu-img Differences between Snapshots

Internal

• qemu-img

Overview

This is a procedure that can be used to list differences between qemu-img snapshots. It is probably not optimal, and there are probably tools that do this better, but at the time of the writing, I did not know about them.

Procedure

1. Take a snapshot of the filesystem before the event whose effects you want to analyze: qemu-img snapshot -c ...

2. Trigger the event.

3. Save the state of the filesystem after the event, by cloning the filesystem, with virsh vol-clone, after the VM guest has been shut down:

virsh vol-clone --pool main-storage-pool --prealloc-metadata build-guest.qcow2 build-guest-after.qcow2

4. Revert the original filesystem to the state before the event, by reverting to snapshot, with qemu-img snapshot -a <snapshot-tag> <volume-name>.

5. Optionally, delete the snapshot with qemu-img snapshot -d <snapshot-tag> <volume-name>.

6. Display the difference:

virt-diff -a <image-before> -A <image-after>

virt-diff -a /main-storage-pool/build-guest.qcow2 -A /main-storage-pool/build-guest-after.qcow2  > .../diff.txt

7. Delete the cloned "after" filesystem with virsh vol-delete ...