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Re: [Qemu-devel] [Qemu-block] [RFC] Proposed qcow2 extension: subcluster

From: Alberto Garcia
Subject: Re: [Qemu-devel] [Qemu-block] [RFC] Proposed qcow2 extension: subcluster allocation
Date: Thu, 13 Apr 2017 11:05:49 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Thu 13 Apr 2017 11:02:10 AM CEST, Kevin Wolf <address@hidden> wrote:
> I think you still need it if you don't want to look at the refcount
> blocks for every write. When you take an internal snapshot, you just
> increase the refcount of the L2 tables at first and keep the contents
> the same, including the subcluster information. On the first write to
> the cluster, like with normal images you need to copy the whole
> cluster, and whether this is the case is determined with the COPIED
> flag. (The copy can actually keep unallocated/zeroed subclusters that
> way, but data has to be copied for the whole cluster.)
> Of course, in all cases the COPIED flag is just an optimisation
> because it's always possible to look at the refcount blocks, but I
> don't think there is any difference between subclustered and
> traditional images in this respect.

Exactly :)


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