Re: [Qemu-devel] [PATCH 2/2] sheepdog: support 'qemu-img snapshot -a'

[Kevin Wolf <kwolf@redhat.com>]

Am 06.06.2013 um 15:09 hat Liu Yuan geschrieben:
> On 06/06/2013 08:46 PM, Kevin Wolf wrote:
> > Am 06.06.2013 um 13:57 hat Liu Yuan geschrieben:
> >> Just call sd_create_branch() to rollback the image is good enough
> >>
> >> Cc: qemu-devel@nongnu.org
> >> Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
> >> Cc: Kevin Wolf <kwolf@redhat.com>
> >> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> >> Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> >> ---
> >>  block/sheepdog.c |    8 +++++---
> >>  1 file changed, 5 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/block/sheepdog.c b/block/sheepdog.c
> >> index 94218ac..cb5ca4a 100644
> >> --- a/block/sheepdog.c
> >> +++ b/block/sheepdog.c
> >> @@ -2072,9 +2072,11 @@ static int sd_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
> >>      }
> >>  
> >>      if (!s->inode.vm_state_size) {
> >> -        error_report("Invalid snapshot");
> >> -        ret = -ENOENT;
> >> -        goto out;
> >> +        /* qemu-img asks us to rollback, we need to do it right now */
> >> +        ret = sd_create_branch(s);
> >> +        if (ret) {
> >> +            goto out;
> >> +        }
> >>      }
> > 
> > I'm not sure how snapshots work internally for Sheepdog, but it seems
> > odd to me that you need to do this only for disk-only snapshots, but not
> > when the snapshot has VM state. (Also, note that 'qemu-img snapshot -a'
> > works on images with a VM state, so the comment doesn't seem to be
> > completely accurate)
> > 
> > Would you mind explaining to me how this works in detail?
> > 
> 
> Hmm, the original code isn't written by me and this snapshot mechanism
> exists since day 0. I just hacks it to work now. So I'll try to explain
> on my understanding.
> 
> When we do a snapshot such as 'savedvm' or 'qemu-img snapshot', the
> active vdi is snapshotted and marked as snapshot and a new vdi is
> created as copy-on-write on the previous active vdi, then this new vdi
> becomes active vdi. For e.g,
> 
> As1 --> As2 --> A
> 
> We take snapshot of vdi A twice, tagged s1 and s2 respectively. I guess
> this is quit similar to qcow2 snapshots, only inode object with a bitmap
> is created.
> 
> So when we 'loadvm' or 'qemu-img snapshot -a' to A, current logic just
> handle 'loadvm', that .bdrv_snapshot_goto only reloads inode object,
> that is, for e.g, we 'savevm s1', and mark it as snapshot, the chain
> would like
> 
> As1 --> As2 --> A
>  |
>  v
>  just reload As1's inode object
> 
> Only when the write comes from VM, we do the following stuff
>  - delete active vdi A
>  - created a new inode based on the previously reloaded As1's inode

Thanks, this is the part that I missed.

I'm not sure however why the actual switch is delayed until the first
write. This seems inconsistent with qcow2 snapshots.

Do you know if there is a reason why we can't always do this already
during bdrv_snapshot_goto()?

> The chain will look like:
> 
> As1 --> As2
>  |
>  V
>  A
> 
> This is how sheepdog handles savevm/loadvm.
> 
> So for 'qemu-img snapshot -a', we should create the branch in the
> .bdrv_snapshot_goto.
> 
> As you pointed out, we need to consider vm state even for 'snapshot -a',
> so I need to rework the patch 2/2.

Yes, the presence of VM state is independent from whether you're using
qemu-img or loadvm. And it actually goes both ways: qemu-img can revert
to snapshots that have a VM state, and loadvm can be used with images
that don't have a VM state (if you have multiple images, only one of
them has the VM state).

Kevin