Re: [Qemu-block] [Qemu-devel] [PATCH for-2.6] block: Forbid I/O throttling on nodes with multiple parents for 2.6

[Eric Blake]

On 04/04/2016 09:26 AM, Kevin Wolf wrote:
> As the patches to move I/O throttling to BlockBackend didn't make it in
> time for the 2.6 release, but the release adds new ways of configuring
> VMs whose behaviour would change once the move is done, we need to
> outlaw such configurations temporarily.
> 
> The problem exists whenever a BDS has more users than just its BB, for
> example it is used as a backing file for another node. (This wasn't
> possible in 2.5 yet as we introduced node references to specify a
> backing file only recently.) In these cases, the throttling would
> apply to these other users now, but after moving throttling to the
> BlockBackend the other users wouldn't be throttled any more.
> 
> This patch both prevents making new references to a throttled node as
> well as using monitor commands to throttle a node with multiple parents.
> 
> Compared to 2.5 this changes behaviour in some corner cases where
> references were allowed before, like bs->file or Quorum children. It
> seems reasonable to assume that users didn't use I/O throttling on such
> low level nodes. With the upcoming move of throttling into BlockBackend,
> such configurations won't be possible anyway.
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block.c    | 7 +++++++
>  blockdev.c | 7 +++++++
>  2 files changed, 14 insertions(+)

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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