[sheepdog] consistency problem when replicating to multi-replica

[Hitoshi Mitake]

At Mon, 05 Jan 2015 09:12:27 +0800,
guping wrote:
> 
> Thanks Hitoshi,
> 
> I just began to evaluate sheepdog for our cloud deployment. I will try 
> to reproduce the problem when I have enough time. If some clue, I will 
> let you know.

Thanks for your interest :) Please let us know if you find problems.

> 
> A question, does the IO stack components higher than sheepdog, such as 
> IO queue in VM, qemu block driver, etc, provide the necessary serialization?

I think these components doesn't have to provide the serialization
functionality. sheepdog (including QEMU driver) must mock the
sematntics of hard drives. If sheepdog allows the case of divergent
objects, it means sheepdog cannot preserve the semantics (two or more
continuous read requests to same location can return different
data). So the serialization should be provided by sheepdog.

Thanks,
Hitoshi

> 
> 在 15/1/5 上午8:52, Hitoshi Mitake 写道:
> > At Sun, 04 Jan 2015 23:27:15 +0800,
> > guping wrote:
> >>
> >> Sheepdog send write request to all replica simultaneously to do the
> >> replication. Which I think could lead to divergent content in replicas.
> >>
> >> Imagine a scenario, there are two write requests A & B to update the
> >> same offset in a 2-replicas object. If a qemu client send A and B to a
> >> gateway concurrently, the gateway will send A and B to the same set of
> >> replicas simultaneously. Which could not guarantee the same sequence to
> >> apply A and B, you can apply A then B in a replica and apply B then A in
> >> another replica, then you get two request return successfully but not
> >> the same data in the replicas.
> >>
> >> Is this a consistency problem in sheepdog? How can we deal with it?
> >
> > Hi Gu,
> >
> > Currently, the above problem is prevented for a case of copy on write
> > requests (first write to an object). Look at
> > check_simultaneous_create() of sheepdog driver in qemu. Multiple COW
> > requests for a single object is serialized for not losing written
> > information illegally.
> >
> > But, the driver doesn't do the serialization for ordinal write
> > request. As you say, there might be a possibility of divergent
> > objects. Could you produce the problem with artificial workload?
> >
> > Thanks,
> > Hitoshi
> >
> >>
> >> --
> >> Thanks,
> >> Gu Ping
> >> --
> >> sheepdog mailing list
> >> sheepdog at lists.wpkg.org
> >> https://lists.wpkg.org/mailman/listinfo/sheepdog
> >
> 
> -- 
> Thanks,
> Gu Ping
> 
> 
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