[Qemu-devel] QCOW rebases on running VMs

[Alex Bligh]

This question concerns the safety of rebases on backing disks whilst a VM is 
running from a writing disk.

Assume that we have the following structure:

    QEMU ------->   Writing disk
                    ....
                    backing disk N
                    ....
                    backing disk 1
                    ....
                    backing disk 0

If QEMU is not running, I can happily use 'qemu-img rebase' to merge backing 
disks 1 ... N (i.e. so backing disk N is now based on backing disk 0).

Is it safe to do this whilst the server is running using the writing disk which 
is based on backing disk N?

It is attractive to think it might be, as even if QEMU has a stale view of the 
contents of backing disk N, backing disks N-1 to 1 (inclusive) will still be 
present, QEMU will still keep an fd open for them, and it will (thus) still be 
able to read the correct sectors from them.

If not, do I need to use QMP magic instead?

-- 
Alex Bligh