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Re: [Qemu-devel] [v3 03/12] block: Add overlay BDS pointer into the Bloc
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [v3 03/12] block: Add overlay BDS pointer into the BlockDriverState struct |
Date: |
Fri, 30 May 2014 10:00:09 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 |
On 05/30/2014 09:35 AM, Jeff Cody wrote:
> Now that node-names can reference an individual BlockDriverState without
> needing to navigate downwards from the 'device' level, in order to find
> the top-most image (active layer) we need a pointer to the overlay of a
> BDS.
>
> This will allow QMP commands to reference an image solely by its
> node-name, without also needing to pass in the corresponding 'device'
> string.
>
> This also adds a helper function to set the overlay pointer that is, for
> now, trivial. But since we recently moved away from open coding
> bs->backing_hd assignment, we should probably also refrain from setting
> bs->overlay directly.
>
> Signed-off-by: Jeff Cody <address@hidden>
> ---
> block.c | 14 ++++++++++++++
> include/block/block.h | 1 +
> include/block/block_int.h | 1 +
> 3 files changed, 16 insertions(+)
Question - can one bds ever belong to more than one chain? That is,
could I create a guest that uses:
/ disk1
base <
\ disk2
for two disks in parallel, but where I use blockdev referencing to pass
in a single readonly copy of base to both chains rather than the default
of opening base twice? If so, then what does the overlay field get set
to for base: disk1 or disk2? On the other hand, if base truly is
shared, it's non-deterministic which device is meant when using just the
node-name for the shared base bds. So maybe the user gets what they
deserved. Or maybe it argues that doing a bds lookup for a bds that has
more than one device chain returns a failure unless the device chain was
passed as well (but that may mean reference counting or even tracking an
array of overlays).
But if a bds can be in at most one chain at a time, then your patch
makes sense.
Reviewed-by: Eric Blake <address@hidden>
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- [Qemu-devel] [v3 00/12] Modify block jobs to use node-names, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 04/12] block: add helper function to find the active layer of any BDS, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 05/12] block: simplify bdrv_find_base(), Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 06/12] block: make 'top' argument to block-commit optional, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 07/12] block: Accept node-name arguments for block-commit, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 08/12] block: extend block-commit to accept a string for the backing file, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 10/12] block: add backing-file option to block-stream, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 09/12] block: add ability for block-stream to use node-name, Jeff Cody, 2014/05/30
- [Qemu-devel] [v3 11/12] block: Add QMP documentation for block-stream, Jeff Cody, 2014/05/30