[Qemu-devel] Re: [RFC][PATCH 2/2] qcow2: Use bdrv_(p)write_sync for metadata writes

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Jun 17, 2010 at 3:39 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 17.06.2010 16:19, schrieb Stefan Hajnoczi:
>> On Thu, Jun 17, 2010 at 1:03 PM, Kevin Wolf <kwolf@redhat.com> wrote:
>>> Use bdrv_(p)write_sync to ensure metadata integrity in case of a crash.
>>
>> Any performance numbers?  This change is necessary for correctness but
>> I wonder what the performance impact is for users.
>
> No numbers yet, but as you say we need to do it anyway. It should
> definitely be better than any other option that I can think of
> (cache=writethrough or some O_DIRECT|O_DSYNC mode) in that it only hurts
> performance when metadata is actually changed. As long as we only write
> guest data, there is no difference.
>
> Making it a barrier instead of a flush would probably be better, have
> you already had a look at this since we talked about it?

No I haven't seen a userspace barrier solution.  I'm not actively
working on that at the moment but am keeping an eye out for solutions.
 I'm not sure what to make of sync_file_range(2), the man page
suggests it cannot be relied on since only already allocated blocks
will be flushed appropriately.  Filesystem metadata changes are not
flushed, which is especially problematic for copy-on-write filesystems
(according to the man page).

Thinking about the performance more, I predict that guest OS
installations will slow down but day-to-day usage (especially
modifying already allocated blocks) will be alright for the reasons
you mentioned.

Stefan