[sheepdog] [PATCH v8 00/19] object reclaim based on generational reference counting

[Hitoshi Mitake]

On Fri, May 23, 2014 at 2:03 PM, Liu Yuan <namei.unix at gmail.com> wrote:
> On Thu, May 22, 2014 at 11:29:59PM +0900, Hitoshi Mitake wrote:
>> At Thu, 22 May 2014 16:54:32 +0800,
>> Liu Yuan wrote:
>> >
>> > On Fri, May 16, 2014 at 12:22:27AM +0900, Hitoshi Mitake wrote:
>> > > The object reclaim doesn't support hypervolume yet. But hypervolume cannot be
>> > > used as a virtual disk (both of qemu and tgt don't support it) currently. And
>> > > the removal of old vdi deletion is acceptable for hypervolume because it doesn't
>> > > support snapshot, etc. So I think this patchset can be applied to the master
>> > > branch.
>> > >
>> > > The same code is pushed to:
>> > > https://github.com/sheepdog/sheepdog/tree/snapshot-object-reclaim
>> > >
>> > > There is a problem which can be caused by discard operation. But the
>> > > problem can be solved as an individual topic. I'll post a patchset for
>> > > it later.
>> > >
>> > > The leak problem was (also) caused by bugs in QEMU's sheepdog
>> > > driver. The fixed version of QEMU driver is here:
>> > >  https://github.com/sheepdog/qemu/tree/inode-sync
>> > > I'll post it to the QEMU list later.
>> > >
>> > > v8:
>> > >  - let COW and snapshot be excluded mutually
>> > >  -- This change introduces limitation that "dog vdi snapshot" must be
>> > >     executed on a same node which execute QEMU. But it is temporal and
>> > >     removed easily.
>> >
>> > What happens if I run 'dog vdi snapsht' on node B while VM runs on node A?
>>
>> Snapshot is created but gref of the snapshot can be inconsistent
>> (depends on timing). But it can be removed easily by reviving VDI lock
>> operation.
>
> Is it possible to exclude this negative effect from this patch set? VDI lock
> operation isn't easy to implement, at least in the near future.
>
> So this open a hole that casual user might destroy the conconsistcy of vdi
> without notice, right? Suppose someone run 'dog vdi snapshot' on different node
> as before, he wouldn't expect side effect.
>
> Even we can't git rid of this limilation in this patchset, we should warn people
> who try to do it programatically.

Removing the limitation from this patchset is hard. But current
snapshot feature in the master branch has similar problems. Because
sheepdog doesn't provide a mechanism for serializing updates of inode
objects.

This patchset solves the problem partially. If users execute "dog vdi
snapshot" on a host which executes a QEMU process, updates of inode
objects are seriealized. It cannot be achieved by the original
snapshot feature. So updating documents for this problem and warn
operators about this problem would be a reasonable way.

In addition, I discussed with Kazutaka-san about the vdi locking
feature and found a smart way of implementation. It can be achieved in
the near future.

Thanks,
Hitoshi