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Re: [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for back

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup
Date: Thu, 9 Nov 2017 10:33:06 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/09/2017 08:16 AM, Vladimir Sementsov-Ogievskiy wrote:
> This is needed to implement image-fleecing scheme, when we create
> a temporary node, mark our active node to be backing for the temp,
> and start backup(sync=none) from active node to the temp node.
> Temp node then represents a kind of snapshot and may be used
> for external backup through NBD.
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> What was the reason to abandon non-root nodes?

I think the original restriction was that we didn't know all the
implications to having multiple readers to an intermediate node, so it
was easier to prevent it with plans to add it later than to add it up
front and deal with the fallout.  But I think that now we are
sufficiently versed in handling BDS trees with multiple readers, with
proper op-blocking in place; so you are right that we can probably
support it now.

However, I'm a bit worried that there is no documentation change about
this in a .json QAPI file, nor any easy way to introspect via QMP
whether a particular qemu implementation supports this (short of trying
it and seeing whether it works).  I'm also thinking that this is 2.12
material, unless we can prove it is fixing a bug for 2.11 that was not
previously present.

>  blockdev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/blockdev.c b/blockdev.c
> index 56a6b24a0b..d0a5a107f0 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -3345,7 +3345,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, 
> BlockJobTxn *txn,
>          backup->compress = false;
>      }
> -    bs = qmp_get_root_bs(backup->device, errp);
> +    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>      if (!bs) {
>          return NULL;
>      }

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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