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From: | Paolo Bonzini |
Subject: | Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes |
Date: | Mon, 24 Feb 2014 13:10:58 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 |
Il 24/02/2014 13:07, Kevin Wolf ha scritto:
> Yeah, that's why I wrote "or should be". Those are the intended > semantics of bdrv_co_write_zeroes without BDRV_REQ_MAY_UNMAP: always > allocate a cluster that will read as zeroes (allocating even if it > does not necessarily write the zeroes).Which would mean that there is no way to say "give me zeroes, and do it in the cheapest way possible". Because that would be to leave the allocation status as it is and just toggle the zero bit.
If bdrv_co_write_zeroes is SCSI's "WRITE SAME without UNMAP", then it must allocate. I think "give me zeroes and do it in the cheapest way possible" is BDRV_REQ_MAY_UNMAP (which *may* unmap but it doesn't have to).
That said, the really expensive part of unmapping is probably re-allocating the clusters on subsequent writes. The unmap itself isn't that expensive (even if you have to flush the refcount blocks before the L2 tables), is it?
Paolo
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