Re: [libvirt] PATCH: Disable QEMU drive caching

[Anthony Liguori <anthony codemonkey ws>]

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
>