[net.micro] shared file-server == time-sharing again

henry (02/28/83)

People who are constructing workstation networks with diskless machines
and shared disk servers should carefully consider two facts:

	1. Unix shows a very strong tendency to run disk-bound.  It would
	seem reasonable to assume that any sophisticated system will share
	this characteristic, in the absence of convincing evidence to the
	contrary.

	2. A shared file server will show the same sort of degradation
	of response with increasing load as a time-sharing machine does:
	the same mechanisms (contention for disk heads, etc.) are at work.

Considering that accessing a file over a network is not as fast as getting
to it on a (well-designed) local disk, this does not augur all that well
for diskless workstations.  Nobody disputes the usefulness of shared file
servers for bulk storage, but the performance implications of doing ALL
your disk i/o that way clearly need careful consideration.

To give credit where it is due:  as far as I know, the first person to
realize the Unix performance implications of shared disk servers (as opposed
to the general usefulness of local disks for performance) was Tom Duff of
Lucasfilm's computer graphics lab.  His prediction has been confirmed
there, and they are now planning to put a local disk on each of their SUNs
for exactly this reason.  Tom recently lost the local disk on his SUN
temporarily, and says that the loss in performance is considerable.  His
estimate is that it takes three times as long to get to a file over a 10-Mb
Ethernet as it does on a local disk (with the same type of disk in both
places, I think).

					Henry Spencer
					U of Toronto

rpk (03/08/83)

It is very true that having a local paging disk helps out a lot.
Here at MIT, the experience with Lisp Machines has been that file access
over the CHAOSNet (roughly like a 4MHz Ethernet) is actually just as fast,
or even faster, than access to local files (even when the file server
itself is a timesharing system).  Most of the advantages of having paging
segments shared on a remote computer can be simulated with the appropiate
net protocols.

``Bob'' UUCP: ...decvax!genradbo!mitccc!rpk
        ARPA: RpK @ MC