On Mon, Sep 14, 2009 at 10:41:51AM -0400, Mark Mielke uttered: > On 09/14/2009 08:27 AM, Alex Craven wrote: >> Still, in spite of the local read-subvolume and caching, successive read >> operations on the same (tiny) file, well within the cache-timeout period >> always take a minimum time equivalent to four round trips to complete. >> > > The read subvolume only helps on read(). For every other request, such > as open(), all of the replicas get involved. I think you are looking for > the dr sync functionality they list on the roadmap for GlusterFS 2.2. The dr sync is a slightly different approach -- I'd still hoped for bidirectional replication in this case, just not necessarily instantaneously. Still, this looks like something I'll need in future, so thanks! For now I think something like inotify+rsync will probably serve me better... pity, I liked gluster in my brief encounter with it. >> Can anyone explain why this is? I understand there may be one transaction >> to validate the cache, but what else is going on here? Furthermore, is it >> possible to defer/disable this re-validation (within a timeout) >> altogether?luster.org/cgi-bin/mailman/listinfo/gluster-users >> > > Not that I know of - and I think there would be many gotchas of doing > such a thing - however, for certain use cases, it would be kind of cool > if they could relax the restrictions and/or move operations such as > consistency checks and self-heal to the background to be run > asynchronously. For example, if I am doing open(..., O_RDONLY), read(), > close(), then I may not care about the state of other volumes, as long > as I see a consistent snapshot that doesn't break half-way through my > read()s... Yeah, this is exactly what I was hoping it could do. I'll see how feasible it is to add something like this in... At any rate, I still don't really see why this consistency check requires four round trips? I wonder if this could be 'pipelined' somehow? In terms of the snapshot possibly breaking half way though a read, is this really any different from a local file being updated by another process mid-read? -- Alex Craven krei at it.net.au
