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