Re: [Qemu-devel] [PATCH] block: Make op blockers recursive

[Eric Blake <eblake@redhat.com>]

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On 09/10/2014 02:54 AM, Fam Zheng wrote:

>> Let's think of a situation that recursive blockers protect but
>> backing_blocker does not:
>>
>> a <- b <- c <- d
>>
>> c is the backing file and is therefore protected by the op blocker.
>>
>> The block-commit command works with node-names, however, so we can
>> manipulate any nodes in the graph, not just the topmost one.  Try this:
>>
>> block-commit d
>> block-commit b
>>
>> I haven't checked yet but I suspect it will launch two block-commit jobs
>> on the same partial chain (that's a bad thing because it can lead to
>> corruption).
> 
> 1) Does block-commit work with node-names already? In other words, is
>    block-commit b possible now? I only see drive-mirror works with it, but not
>    drive-backup, block-mirror or block-commit.

IIRC, Jeff Cody proposed patches for qemu 2.1 that would have done this,
but we dropped them for that release in order to get the recursive
blockers sorted out first.

 >
> 2) Regardless of the answer to 1), I think we could use a similar approach as
>    drive-backup here: split BLOCK_OP_TYPE_COMMIT to
>    BLOCK_OP_TYPE_COMMIT_{SOURCE,TARGET}, and only unblock
>    BLOCK_OP_TYPE_COMMIT_TARGET in bdrv_set_backing_hd.

In that earlier thread, Jeff had some ideas that it is not so much the
operation name that should be the blocker, but the lower-level action(s)
implied by each operation (read metadata, write metadata, read image,
write image)

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


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