Re: [Qemu-devel] [PATCH 1/5] block: Remove copy-on-read from bdrv_move_feature_fields()

From: Eric Blake <eblake@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org
Cc: qemu-devel@nongnu.org, jsnow@redhat.com, armbru@redhat.com,
	mreitz@redhat.com
Subject: Re: [Qemu-devel] [PATCH 1/5] block: Remove copy-on-read from bdrv_move_feature_fields()
Date: Mon, 14 Mar 2016 09:51:02 -0600	[thread overview]
Message-ID: <56E6DDE6.7040402@redhat.com> (raw)
In-Reply-To: <1457970292-12291-2-git-send-email-kwolf@redhat.com>

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On 03/14/2016 09:44 AM, Kevin Wolf wrote:
> Ever since we first introduced bdrv_append() in commit 8802d1fd ('qapi:
> Introduce blockdev-group-snapshot-sync command'), the copy-on-read flag
> was moved to the new top layer when taking a snapshot. The only problem
> is that it doesn't make a whole lot of sense.
> 
> The use case for manually enabled CoR is to avoid reading data twice
> from a slow remote image, so we want to save it to a local overlay, say
> an ISO image accessed via HTTP to a local qcow2 overlay. When taking a
> snapshot, we end up with a backing chain like this:
> 
>     http <- local.qcow2 <- snap_overlay.qcow2
> 
> There is no point in doing CoR from local.qcow2 into snap_overlay.qcow2,
> we just want to keep copying data from the remote source into
> local.qcow2.
> 
> The other use case of CoR is in the context of streaming, which isn't
> very interesting for bdrv_move_feature_fields() because op blockers
> prevent this combination.

Your arguments make sense.

> 
> This patch makes the copy-on-read flag stay on the image for which it
> was originally set and prevents it from being propagated to the new
> overlay. It is no longer intended to move CoR to the BlockBackend level.
> In order for this to make sense, we also need to keep the respective
> image read-write.
> 
> As a side effect of these changes, creating a live snapshot image (as
> opposed to using an existing externally created one) on top of a COR
> block device works now. It used to fail because it tried to open its
> backing file both read-only and with COR.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c    | 2 --
>  blockdev.c | 7 +++++--
>  2 files changed, 5 insertions(+), 4 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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