Le Tuesday 17 Sep 2013 à 14:44:13 (+0200), Stefan Hajnoczi a écrit :
> Hi Benoit,
> Kevin and I had a chance to chat face-to-face and we discussed what
> concrete changes are necessary to merge quorum (without solving all the
> other block layers problems at once).
>
> I think quorum could be merged relatively quickly (and without massive
> BlockFilter investments) by changing the following:
>
> 1. Defining children on the command-line
>
> Existing "filter" drivers use the protocol filename to embed their
> children, for example the blkverify driver. This is a big hack because
> we have no proper syntax or escaping for the embedded drive definitions
> in the file= option.
>
> This was one of the main arguments against merging quorum. Now that
> Kevin has implemented driver-specific open options (see
> block/qcow2.c:qcow2_runtime_opts), it is possible for the quorum driver
> to open children specific on the command-line:
>
> -drive if=none,file.driver=quorum,format=raw,\
> file.children0.file=/nfs1/test.qcow2,\
> file.children1.file=/nfs2/test.qcow2,\
> file.children2.file=/nfs3/test.qcow2
>
> Here is pseudo-code for quorum_open(). It extracts the children from
> the command-line and opens BlockDriverStates for them:
>
> def quorum_open(qdict):
> for key in qdict:
> # Parse out child index
> match = key.match('^children(\d+)\.')
> if not match:
> continue
> idx = match.group(0)
>
> # block.c:extract_subqdict() already lets you extract a new
> # qdict with the fields starting with a prefix. You could make
> # this function common.
> child_dict = extract_subqdict(qdict, 'children%d.' % idx)
>
> # Finally open the child with its specific options
> children[idx] = bdrv_open(child_dict)
>
> After doing this we no longer need to embed the children into the file=
> option as a string and driver-specific options can be used for the
> children.
>
> 2. Prevent external snapshots
>
> The user may wish to snapshot above or below the quorum BDS. There is
> no syntax to express the BDS tree node where snapshotting should happen.
> Implementing this fully is a distraction and requires additional work.
>
> For quorum it is simplest to add a BDS interface so block drivers can
> forbid snapshots. This is called recursively on the whole BDS tree, so
> quorum can veto the snapshot operation. An error is returned saying the
> device does not support snapshots.
>
> In the future this limitation can be lifted but for the near-term this
> is a simple step to make quorum mergable.
>
> With these changes quorum should be mergable. Implementing full
> BlockFilter support is not necessary yet but we can discuss it in
> separate thread, if you want.
Thanks Stefan !!
I will do these changes to make quorum mergeable and yes I am still interested
in writing BlockFilter support given I have a plan to do so.
Best regards
Benoît
>
> Stefan
>
