Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations

[Richard Laager]

On Sat, 2012-03-24 at 16:30 +0100, Christoph Hellwig wrote:
> On Wed, Mar 14, 2012 at 01:14:18PM +0100, Paolo Bonzini wrote:
> > 
> > Note that the discard granularity is only a hint, so it's really more a
> > maximum suggested value than a granularity.  Outside of a cluster
> > boundary the format would still have to write zeros manually.
> > 
> > Also, Linux for example will only round the number of sectors down to
> > the granularity, not the start sector.
> 
> Which really is more of a bug than a feature.  The current discard
> implementation in Linux is still very poor.

The disk protocols do not require the granularity to be respected. It is
*only a hint*. Therefore, QEMU has to handle non-aligned discards. It
doesn't really matter what we wish was the case; this is the reality.

-- 
Richard

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