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

From: Alberto Garcia <berto@igalia.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>, Eric Blake <eblake@redhat.com>,
	qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>,
	qemu-block@nongnu.org
Subject: Re: [Qemu-devel] [Qemu-block] [RFC] Proposed qcow2 extension: subcluster allocation
Date: Thu, 13 Apr 2017 11:05:49 +0200	[thread overview]
Message-ID: <w51vaq8vfrm.fsf@maestria.local.igalia.com> (raw)
In-Reply-To: <20170413090210.GA5095@noname.redhat.com>

On Thu 13 Apr 2017 11:02:10 AM CEST, Kevin Wolf <kwolf@redhat.com> 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 :)

Berto