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Date: Fri, 10 Sep 2010 17:24:15 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format
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To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

  On 09/10/2010 05:12 PM, Christoph Hellwig wrote:
> On Fri, Sep 10, 2010 at 05:05:16PM +0300, Avi Kivity wrote:
>>> Note that ATA allows simply ignoring TRIM requests that we can't handle,
>>> and if we don't set the bit that guarantees TRIMed regions to be zeroed
>>> we don't even have to zero out the regions.
>> It would be nice to support it.  TRIM is important to recover space,
>> otherwise images grow and grow and there's no point in using a sparse
>> format in the first place.
> Sure.  But supporting to tiny TRIM requests doesn't make sense.  That
> is the same behaviour we see from real life SSDs, btw.  If the request
> is smaller than their erase block size or whatever internal structure
> they use to track allocations it will not actually free space.  On some
> of the lesser quality consumer SSDs the sectors won't even be zeroed
> even if they claim so in the ATA IDENTIFY response.

Okay.  Let's concentrate on those UNMAP requests that seem better designed.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.