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


From: Christoph Hellwig
Subject: Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations
Date: Sat, 24 Mar 2012 16:33:07 +0100
User-agent: Mutt/1.5.17 (2007-11-01)

On Wed, Mar 14, 2012 at 01:49:48PM +0100, Paolo Bonzini wrote:
> It does make the distinction.  "I don't care" is UNMAP (or WRITE
> SAME(16) with the UNMAP bit set); "I want to have zeroes" is WRITE
> SAME(10) or WRITE SAME(16) with an all-zero payload.

But once the taget sets the unmap zeroes data bit even UNMAP / WRITE SAME with
unmap have to zero all blocks touched.

Also strictly speaking even the WRITE SAME with unmap requires a payload
of zeroes, even if most targets don't bother to check for this.




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