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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.


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