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

[Daniel P. Berrange]

On Sat, Mar 24, 2012 at 04:29:26PM +0100, Christoph Hellwig wrote:
> On Wed, Mar 14, 2012 at 01:01:35PM +0100, Kevin Wolf wrote:
> > Paolo mentioned a use case as a fast way for guests to write zeros, but
> > is it really faster than a normal write when we have to emulate it by a
> > bdrv_write with a temporary buffer of zeros? On the other hand we have
> > the cases where discard really means "I don't care about the data any
> > more" and emulating it by writing zeros is just a waste of resources there.
> 
> On raw a real discard also will destroy the preallocation, which might
> absolutely kill your performance on filesystems.

This suggests that there be a new command line param to '-drive' to turn
discard support on/off, since QEMU can't reliably know if the raw file
it is given is intended to be fully pre-allocated by the mgmt app.

Regards,
Daniel
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