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Re: [Gluster-devel] [RFC] A new caching/synchronization mechanism to spe


From: Ira Cooper
Subject: Re: [Gluster-devel] [RFC] A new caching/synchronization mechanism to speed up gluster
Date: Wed, 5 Feb 2014 19:16:07 -0500

Yep... this is an area I am very interested in, going forwards.

Especially sending messages back, we'll need that for any caching/leasing/oplock/whatever we call it type protocols.

Keep me in the loop, and I'll keep tracking the list.  (I'm already on the list.)

I'm also ira on freenode if you want to find me.

Thanks,

-Ira / ira@(samba.org|redhat.com)


On Wed, Feb 5, 2014 at 6:24 PM, Anand Avati <address@hidden> wrote:
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 <address@hidden> 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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