10-16-04 07:45 AM
"George Orwell" <nobody@mixmaster.it> wrote in message
news:aeec234d024a23c8b2609c011a2dacbc@mi
xmaster.it...
> I have servers and storage at two sites, with an FC bridge over an ip WAN
in
> between them to create a SAN.
> Now I would like to run a distributed filesystem on servers at both sites,
> accessing the same storage.
>
> For redundancy reasons, I would also like to replicate all the storage so
both
> sites have a copy of the data at all times. If I chose synchronous
replication,
> I would expect each site to be able to use the local copy for reads,
giving
> me local read speeds.
>
> Does anyone know of a filesystem/SAN appliance combination that can give
me
> this ?
The short answer is, I don't. Most storage-level 'synchronous replication'
mechanisms were not set up to allow any (coordinated) access at all to the
'remote' replica, because they were created back when storage-level
replication was usually done with a 'passive' remote copy meant only to take
over if the primary copy became inaccessible.
Some distributed systems (VMS's comes to mind) will cheerfully optimize
reads to target the nearest replica if you make them aware that this is
desired. But that facility is part of VMS's system-software-controlled
mirroring facilities rather than of an underlying hardware array doing its
own remote mirroring (though its HSx controllers may offer similar
facilities nowadays).
Finally, even if you find some hardware set up to allow concurrent read
activity at both replicas, unless you are using a distributed file system
that optimizes its coordination (locking) mechanisms across both sites so
that at least when possible locks are managed at the end that's actually
using the associated data (again, VMS comes to mind here), you still may
have inter-site lock traffic slowing things down even if the actual read
accesses are performed locally (though that will only affect latency, rather
than create the serious inter-site bandwidth demands that reading only at
the 'primary' site might).
Take a look at Lustre: it created a clone of the VMS distributed lock
manager, and might (or might not) offer what you're looking for (though in a
distributed-file-server-style environment rather than a conventional SAN).
AIX's GPFS might too (IBM cloned the VMS lock manager about 10 years ago,
primarily to support Oracle Parallel Server if I understand correctly, and
may later have used it with GPFS in a manner you might find useful).
But last I knew (and things might have changed since), most SAN file systems
thought they were doing pretty well just coordinating shared access to
storage from multiple hosts, without getting into finer points such as
optimizing disaster-tolerant configurations where both sites were active.
So if you find out differently, please let us know.
- bill
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