[Gluster-devel] Can I bring a development idea to Dev's attention?

[Gordan Bobic]

On 09/26/2010 12:54 AM, Ed W wrote:
> On 24/09/2010 09:42, Gordan Bobic wrote:
>> This sounds remarkably similar to how DLM in GFS works. It caches file
>> locks, so performance is reasonable where a set of files is only
>> accessed from one of the nodes. Might it be easier to interface with
>> DLM for locking control instead of implementing such a thing from
>> scratch?
>>
>> The bulk of the performance hit comes from ping times, rather than
>> bandwidth issues / writeback caching. Latencies on a LAN are typically
>> 100us on Gb ethernet, vs. a typical RAM latency of 50ns, so call it a
>> 2000x difference. If this overhead on file lock acquisition can be
>> avoided, it'll make a lot more difference than data caching.
>
> You have hit the nub of the issue.
>
> Actually there are two issues here:
>
> 1) it would be excellent to see some kind of optimistic locking (which I
> think is a good generic description of this kind of optimisation)
> implemented in gluster
>
> 2) actually there are some subtleties in implementing this algorithm if
> you relax the restriction that all nodes are reliable, eg what happens
> if the node holding the locking information dies? What happens if the
> server holding the lock dies (and a request turns up at the secondary
> server)? How long do you wait before you decide to do something to break
> the deadlock and continue?
>
> Paxos/Fatlease are clever implementations of a locking strategy in the
> face of non robust "bricks". That said, google used a similar strategy
> up until recently and mogile and most other cluster systems still do use
> a non redundant locking strategy, so gluster going down the simple route
> would still largely be "state of the art" - however, a robust, shared
> nothing, redundant lock strategy is just icing on the cake?

I still think it would be worth investigating interfacing with DLM, 
which already does this sort of thing, and is already there. Untill the 
scope for re-using that is investigated, it may better to hold off on 
re-inventing that particular wheel.

Gordan