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Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operat
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero |
Date: |
Mon, 12 Mar 2012 12:04:26 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120209 Thunderbird/10.0.1 |
Am 12.03.2012 11:42, schrieb Avi Kivity:
> On 03/09/2012 06:42 PM, Kevin Wolf wrote:
>>>>
>>>> While (3) can be worked around, the only way around the other two,
>>>> unfortunately, is support in the formats and protocols. We will still
>>>> provide device options to opt out of this, but with raw and qed covered
>>>> (+ qcow2 without backing file, and qcow3 in the future) it should not
>>>> be too bad.
>>>
>>> Can't qcow2 with a backing file also be supported? Zero out the first
>>> cluster, and remember it. The following discards can reuse this zero
>>> cluster, as long as it hasn't been overwritten.
>>
>> qcow2 can't handle clusters that are referenced twice from the same L1
>> table. This would require a reverse lookup to adjust the QCOW_O_COPIED
>> flags in the L2 tables containing the other references.
>
> Don't follow, sorry. What adjustment are you talking about?
>
> If it's a 1->0 transition, is it mandatory to adjust the flag? That is,
> it it legal to have a refcount of exactly one, but have the flag clear?
According to the spec it's illegal and qemu-img check will complain,
too. In practice, I'm not entirely sure if it will cause real corruption
or just unnecessary COWs. I believe it may be the latter.
But it's not worth the trouble anyway when we can have a real zero flag
in qcow3.
Kevin
- [Qemu-devel] [RFC PATCH 05/17] block: pass around qiov for write_zeroes operation, (continued)
- [Qemu-devel] [RFC PATCH 05/17] block: pass around qiov for write_zeroes operation, Paolo Bonzini, 2012/03/08
- [Qemu-devel] [RFC PATCH 12/17] ide/scsi: turn on discard, Paolo Bonzini, 2012/03/08
- [Qemu-devel] [RFC PATCH 01/17] qemu-iotests: add a simple test for write_zeroes, Paolo Bonzini, 2012/03/08
- [Qemu-devel] [RFC PATCH 11/17] ide/scsi: prepare for flipping the discard defaults, Paolo Bonzini, 2012/03/08
- [Qemu-devel] [RFC PATCH 16/17] qemu-io: fix the alloc command, Paolo Bonzini, 2012/03/08
- [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero, Paolo Bonzini, 2012/03/08
[Qemu-devel] [RFC PATCH 08/17] block: kill the write zeroes operation, Paolo Bonzini, 2012/03/08
[Qemu-devel] [RFC PATCH 15/17] raw: add get_info, Paolo Bonzini, 2012/03/08
[Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming, Paolo Bonzini, 2012/03/08
Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming, Stefan Hajnoczi, 2012/03/09