[Gluster-devel] [RFC] A new caching/synchronization mechanism to speed up gluster

[Anand Avati]

Xavi,
Getting such a caching mechanism has several aspects. First of all we need
the framework pieces implemented (particularly server originated messages
to the client for invalidation and revokes) in a well designed way.
Particularly how we address a specific translator in a message originating
from the server. Some of the recent changes to client_t allows for
server-side translators to get a handle (the client_t object) on which
messages can be submitted back to the client.

Such a framework (of having server originated messages) is also necessary
for implementing oplocks (and possibly leases) - particularly interesting
for the Samba integration.

As Jeff already mentioned, this is an area where gluster has not focussed
on, given the targeted use case. However the benefits of extending this to
internal use cases (to avoid per-operation inodelks can benefit many
modules - encryption/crypt, afr, etc.) It seems possible to have a common
framework for delegating locks to clients, and build caching coherency
protocols / oplocks / inodelk avoidence on top of it.

Feel free to share a more detailed proposal if you have have/plan - I'm
sure the Samba folks (Ira copied) would be interested too.

Thanks!
Avati


On Wed, Feb 5, 2014 at 11:27 AM, Xavier Hernandez <xhernandez at datalab.es>wrote:

> On 04.02.2014 17:18, Jeff Darcy wrote:
>
>  The only synchronization point needed is to make sure that all bricks
>>> agree on the inode state and which client owns it. This can be achieved
>>> without locking using a method similar to what I implemented in the DFC
>>> translator. Besides the lock-less architecture, the main advantage is
>>> that much more aggressive caching strategies can be implemented very
>>> near to the final user, increasing considerably the throughput of the
>>> file system. Special care has to be taken with things than can fail on
>>> background writes (basically brick space and user access rights). Those
>>> should be handled appropiately on the client side to guarantee future
>>> success of writes. Of course this is only a high level overview. A
>>> deeper analysis should be done to see what to do on each special case.
>>> What do you think ?
>>>
>>
>> I think this is a great idea for where we can go - and need to go - in the
>> long term. However, it's important to recognize that it *is* the long
>> term. We had to solve almost exactly the same problems in MPFS long ago.
>> Whether the synchronization uses locks or not *locally* is meaningless,
>> because all of the difficult problems have to do with recovering the
>> *distributed* state. What happens when a brick fails while holding an
>> inode in any state but I? How do we recognize it, what do we do about it,
>> how do we handle the case where it comes back and needs to re-acquire its
>> previous state? How do we make sure that a brick can successfully flush
>> everything it needs to before it yields a lock/lease/whatever? That's
>> going to require some kind of flow control, which is itself a pretty big
>> project. It's not impossible, but it took multiple people some years for
>> MPFS, and ditto for every other project (e.g. Ceph or XtreemFS) which
>> adopted similar approaches. GlusterFS's historical avoidance of this
>> complexity certainly has some drawbacks, but it has also been key to us
>> making far more progress in other areas.
>>
>>  Well, it's true that there will be a lot of tricky cases that will need
> to be handled to be sure that data integrity and system responsiveness is
> guaranteed, however I think that they are not more difficult than what
> can happen currently if a client dies or loses communication while it
> holds a lock on a file.
>
> Anyway I think there is a great potential with this mechanism because it
> can allow the implementation of powefull caches, even based on SSD that
> could improve the performance a lot.
>
> Of course there is a lot of work solving all potential failures and
> designing the right thing. An important consideration is that all
> these methods try to solve a problem that is seldom found (i.e. having
> more than one client modifying the same file at the same time). So a
> solution that has almost 0 overhead for the normal case and allows the
> implementation of aggressive caching mechanisms seems a big win.
>
>
>  To move forward on this, I think we need a *much* more detailed idea of
>> how we're going to handle the nasty cases. Would some sort of online
>> collaboration - e.g. Hangouts - make more sense than continuing via
>> email?
>>
>>  Of course, we can talk on irc or another place if you prefer
>
> Xavi
>
>
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