[libvirt] PATCH: Disable QEMU drive caching

Anthony Liguori anthony at codemonkey.ws
Thu Oct 9 14:06:42 UTC 2008

Anthony Liguori wrote:
> Mark McLoughlin wrote:
> And let's take a step back too.  If people are really concerned about 
> this point, let's introduce a sync=on option that opens the image with 
> O_SYNC.  This will effectively make the cache write-through without 
> the baggage associated with O_DIRECT.

I'm starting to slowly convince myself we should always open files with 
O_SYNC.  Barriers should just force ordering within the thread pool.  
posix-aio has no interface for this but we could create one with our own 
thread pool implementation.

Ryan: could you give the following patch a perf-run so we can see how 
this would effect us?


Anthony Liguori

> While I object to libvirt always setting cache=off, I think sync=on 
> for IDE and SCSI may be reasonable (you don't want it for virtio-blk 
> once we implement proper barriers with fdatasync I think).
> Regards,
> Anthony Liguori
>> Cheers,
>> Mark.

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