Re: [Qemu-devel] [PATCH 0/4] export internal snapshot by qemu-nbd

[Kevin Wolf]

Am 17.07.2013 um 16:23 hat Eric Blake geschrieben:
> On 07/17/2013 08:03 AM, Wenchao Xia wrote:
> > This series allow user to read internal snapshot's contents without qemu-img
> > convert. Another purpose is that, when qemu is online and have taken an
> > internal snapshot, let user invoke qemu-nbd to do any thing on it except 
> > write.
> > 
> > This brings two interesting issues:
> > 1 is it safe to let qemu-nbd and qemu access that file at same time?
> 
> Probably not, for the same reason we tell people to not use qemu-img
> while qemu is active on a file.

No, it's not. There's the built-in NBD server, but making internal
snapshots usable with it would require some non-trivial changes in the
block layer and the qcow2 code.

> > I think it is safe, since qemu-nbd is read only. The data will be correct 
> > from
> > qemu-nbd, if qemu does not delete that snapshot when qemu-nbd is running, 
> > and
> > data is flushed to storage after qemu take that snapshot so that qemu-nbd
> > would see the correct data.
> 
> You're making assumptions that qemu won't be touching any metadata in a
> manner in which the read-only qemu-nbd could get confused; I'm not sure
> we are ready to make that guarantee.  I think the export has to be from
> the running qemu process itself, rather than from a second process.

I think a while ago I convinced myself that in practice it does work,
but it's not a guarantee we're making and I won't hesitate to break the
assumption if it's helpful for some feature.

> > 2 should an nbd-server exporting internal snapshot be added in qemu?
> > I think no. Compared with driver-backup, the snapshot, or COW happens
> > in storage level, so it allows another program to read it itself. Actually
> > it should be OK to let another server other than qemu's host, do the
> > export I/O job, if data is flushed.
> 
> Unfortunately, I disagree, and think the answer to this question is yes,
> we need to do the export from within the single qemu process, if we want
> to guarantee safety.

Agreed.

Kevin