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Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operat


From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero
Date: Mon, 12 Mar 2012 12:42:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120216 Thunderbird/10.0.1

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?

-- 
error compiling committee.c: too many arguments to function




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