[Lustre-discuss] MGS - one per site

[Andreas Dilger]

On Jun 26, 2009  16:34 -0400, David Dillow wrote:
> On Fri, 2009-06-26 at 22:05 +0200, Andreas Dilger wrote:
> > In some cases there will of course be a need for multiple MGSes in
> > a single site (e.g. secure and open networks), which is fine as long
> > as clients don't try to mount from multiple MGSes at once.
> 
> What kind of problems could this cause?
> 
> We've had this configuration on both segments of Jaguar for some time
> now with no ill effects that we could attribute to mounting from two
> MGSes at the same time -- well, maybe that is not entirely true, as one
> is an MGS, and the other is a combined MGS/MDS. Some clients are talking
> with up to five separate combined MGS/MDS.

Well, there is only ever a single MGC configured on a client at one time,
so if you have multiple MGSes I would suspect that this will cause the
clients to be evicted from all but the last MGS, and as a result any config
changes made to the first-mounted filesystems will not be seen by the
clients.  This might not be noticable until you make a config change and
half of the clients don't notice e.g. the new OST or similar.

Some time in the future the MGS will also be involved in client recovery
(imperative recovery) because the MGS will be the one transmitting the
health messages between the clients and servers.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.