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Re: [Qemu-devel] [PATCH v3 0/6] block: zero writes

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v3 0/6] block: zero writes
Date: Wed, 21 Dec 2011 16:03:36 +0000

On Wed, Dec 21, 2011 at 4:00 PM, Stefan Hajnoczi
<address@hidden> wrote:
> This series adds an interface for efficient writes when data contains all
> zeros.  It also takes advantage of this new interface by extending the
> copy-on-read feature to perform zero-detection.
> The details of efficient zero representations depend on the image format.  
> This
> series includes a patch for the QED image format to write special "zero
> clusters" that keep the image file compact.  In the future qcow2v3 could also
> support an efficient zero representation.
> The new BlockDriver interface is called .bdrv_co_write_zeroes() and is
> optional.  If the interface is not implemented by a BlockDriver then a regular
> .bdrv_co_writev() operation will be performed.  The public interface is called
> bdrv_co_write_zeroes() and can be tested via the new qemu-io write -z option.
> Copy-on-read is extended to detect zeroes and invoke the
> .bdrv_co_write_zeroes() interface when possible.  As a result we avoid 
> bloating
> the image file if the backing file contains zeroes.
> My motivation for this feature is efficient image streaming.  The destination
> file must stay compact even when copying zeroes from the source file.
> We now only do zero detection for copy-on-read requests, whereas previous
> revisions of this patch series scanned all write requests for zeroes.  The old
> behavior wasted CPU cycles in most cases but we could add a feature to
> explicitly scan guest writes for zeroes in the future, if desired.
> v2:



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