NFS performance modeling info requested

[steve at umiacs.umd.edu]

OK, I'm curious:  what have people Out There done in terms of modeling NFS
performance?  What I'd really like to see is information on the following:

-- for the mythical average fileserver, are there multiple requests
   arriving at the same time, or is it often the case that the server is
   servicing one workstation at a time?

-- related to the above, does a 'packet-train' model apply to NFS?  (See
   Steve Heimlich's recent Usenix paper for info on what a packet train is.)

-- How do different models of client/server organization change the load?
   (For example, how much extra load do diskless workstations cause?  If all
   my workstations have local disks, and I put all my user files on the local
   disk with /usr on the server, how does that change things?  If I put the
   user files on the servers, and /usr on the local disks, what difference
   does that make?)

-- Under different client/server models, which runs out first, the client
   CPU, the server CPU, or the network?  If the packet-train model applies,
   and there are few if any overlapping trains, which loses first?  If there
   is lots of overlap in service requests (because multiple clients are
   banging on the server at the same time, keeping it from doing much in the
   way of sequential reads), how does that change the picture?  How does the
   "user data local" (few writes) versus "system files local" (fewer reads,
   perhaps, but more writes) change things?

-- Under the "user data local" and "system files local" models, are there
   files that are referenced much more frequently than others?

Yes, I know that different disks, network configurations, CPU speeds, etc.
will all strongly influence the results.  I even think I have some answers
to some of these questions, at least for the UMCP CSD and/or UMIACS
configurations and networks.  What I'm looking for is enough data points
(i.e., "my configuration looks like this, and this is what I see") to
begin to build a general model.

It occurs to me that hacking the kernel to record NFS requests and
timestamps is a reasonable way to get a handle on the request arrival
characterization problem, and is probably a reasonable way to get a handle
on the "which files are referenced most" problem.  That seems like an easy
hack, so I might whip that out and see what happens.

There is a masters' student here who is working on a fairly extensive
characterization of NFS client and server loading, but (a) she should know
what previous work has been done in the area, and (b) I'm just downright
curious.  The answers to these questions will strongly influence server
purchasing decisions, and I've got some servers to plan for...

Please reply directly to me, and I'll summarize.  Thanks.

-Steve

Spoken: Steve Miller    Domain: steve at umiacs.umd.edu    UUCP: uunet!mimsy!steve
Phone: +1-301-454-1808  USPS: UMIACS, Univ. of Maryland, College Park, MD 20742