[Lustre-discuss] Lustre client and OST on same machine

[Peter Grandi]

> I have a small lustre setup having 1 MDS, 2 OSS and all of the
> three machine also have lustre client mounted.

Note that running the Lustre client on a Lustre server is not
recommnended because there can be a resource deadlock between
the client and server modules (involving the cache IIRC).

However I suspect that this problem occurs only on OSSes, and I
suspect that it is negligible on the MDS.

Which makes me suspect that running some types of workloads on
the MDS as a client may give some advantages.

> Once the lustre client knows about the stripe information of a
> file, will it directly communicate to OSS?

After fetching the file metadata from (one of) the MDS(es)
Lustre clients always communicate directly with the OSS(es)
involved.  That's the whole point of having distinct metadata
and data servers.

A coarse way of understanding Lustre and similar filesystem
types is to imagine that Lustre clients can "mount" invidual
files (or strips of a file) from an OSS, and the MDS is the
server with the automount maps.

> If yes, is there any optimization possible if lustre client
> learns that the data is in the same machine (when acting as
> OST) ?

Not directly. RPCs may be rather faster locally than over a
network interface.