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Re: [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() inter

From: Christoph Hellwig
Subject: Re: [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() interface
Date: Wed, 21 Dec 2011 17:50:32 +0100
User-agent: Mutt/1.5.17 (2007-11-01)

On Wed, Dec 21, 2011 at 04:00:36PM +0000, Stefan Hajnoczi wrote:
> The ability to zero regions of an image file is a useful primitive for
> higher-level features such as image streaming or zero write detection.
> Image formats may support an optimized metadata representation instead
> of writing zeroes into the image file.  This allows zero writes to be
> potentially faster than regular write operations and also preserve
> sparseness of the image file.
> The .bdrv_co_write_zeroes() interface should be implemented by block
> drivers that wish to provide efficient zeroing.
> Note that this operation is different from the discard operation, which
> may leave the contents of the region indeterminate.  That means
> discarded blocks are not guaranteed to contain zeroes and may contain
> junk data instead.

Most real life discard operations zero the data, and both the ATA and SCSI
spec allow the device to set a bit which gurantees this behaviour.  I think
we also should make these one interface, and if the caller needs it to
actually zero out the discarded blocks it should check if the discard
implementation guarantees that.

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