[Lustre-discuss] Failover Setup MDS/MDT

[Peter Grandi]

>>> On Fri, 20 Jun 2008 09:54:20 -0400, "Brian J. Murrell"
>>> <Brian.Murrell at Sun.COM> said:

>> What would be the proper setup scenario if the above is not
>> possible ? (Avoiding a single point of failure for the MDS)

That's a somewhat naive idea of "Avoiding a single point of
failure", for example if the MDT drives are all of the same
type, they will have the same firmware, design weaknesses, and
that will be a single point of failure.

> Two MDSes each with access to the same storage (the MDT)
> device so that either (but never both at the same time!) can
> mount the MDT.

Well, I'd like to see an argument why a single storage backend
shared by two storage-to-network frontends (often identical ones
running the same software) helps much that much with availability.
I'd rather have fully mirrored storage backends (ideally of very
different types):

> DRBD has the potential to simulate this shared storage by
> mirroring it, but I have no first-hand experience doing so.

Well, DRBD does not merely "simulate this shared storage", it
implements fully mirrored dual storage.

Perhaps it does then logically present them as a single image,
but the important detail is that the storage is duplicated
across two units that can be very different and even in
different locations.

Also, between duplicating the backend storage and duplicating
the frontend MDS I'd rather do the former, as in the MDT case
data safety is rather critical. It is potentially very easy to
put in a new MDS, but recovering and restoring the MDTs is
a rather bigger risk...