Re: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"

[Avi Kivity]

 On 08/24/2010 03:35 PM, Kevin Wolf wrote:
> > I don't know how bad the performance regression is, and how large the
> > integrity risk is.  I'd default towards preserving integrity, but maybe
> > this situation is different.
> I have reports of installations taking like 50 min instead of 14 min. My
> own qemu-io based test goes up from 1 s to 23 s. And I think the winner
> is Michael's image conversion which went up from 30 s to 49 min.
>
> So it's not like we're talking about just some 10 or 20 percent.

Image conversion should be done with cache=unsafe (with an fsync at the 
end to make sure a guest isn't launched with volatile data in the 
cache[1]).  The io test isn't a user workload.

The 14 min -> 50 min regression is pretty bad, but I'm not sure it's bad 
enough to merit risking user data.

[1] or better, when launching a guest with cache!=unsafe start with an 
fsync to make sure it's on disk.

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