[libvirt] PATCH: Disable QEMU drive caching
Anthony Liguori
anthony at codemonkey.ws
Wed Oct 8 16:53:14 UTC 2008
Daniel P. Berrange wrote:
> On Wed, Oct 08, 2008 at 10:51:16AM -0500, Anthony Liguori wrote:
>
>> A great deal of virtualization users are doing some form of homogeneous
>> consolidation. If they have a good set of management tools or
>> sophisticated storage, then their guests will be sharing base images or
>> something like that. Caching in the host will result in major
>> performance improvements because otherwise, the same data will be
>> fetched multiple times.
>>
>
> NB, this has no impact on caching of backing files - QEMU masks out
> the O_DIRECT flag when opening the backing file
It doesn't mask out O_DIRECT, it just doesn't pass any flags to the
backing file when it opens it. IMHO, this is a bug.
Regards,
Anthony Liguori
> - so in a shared
> master image scenario, all reads for the shared file will still be
> cached, only write5Cs to the cow file are impacted.
>
> Daniel
>
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