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Re: [libvirt] PATCH: Disable QEMU drive caching

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.


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.


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