jqj@uoregon.UUCP (JQ Johnson) (04/09/88)
One major advantage of Locus that has been mentioned but not, I think, given the attention it deserves is file replication. As systems become more and more complex, the work a person does starts more and more to be dependendent on the simultaneous availability of numerous files. Meanwhile, as remote file systems become larger and more complex, the number of file servers in a cluster increases and it becomes likely that the user will depend on the availability of several file servers simultaneously. On a typical NFS sun cluster I depend on the machine I'm swapping to (presumably the same machine that has my /usr.MC68020 on!), the machine that has my home directory on it, the machine that has the project files I'm editing on it, and perhaps the machine that has the experimental tools I'm using. Assuming 95% uptime for each server, then at least one will be down 1/5 of the time! Worse, there are some files that it is important to have available at all times, but that you may not want to maintain multiple copies of (e.g. /etc/passwd or /.login). SUN solves this problem using a replicated YP plus rdist, but that doesn't generalize well to frequently updated non-database files that one might want "the same" on all workstations. A solution to both problems is to make the files remote, and put them on a replicated file server. A project at Cornell last year built a simple replicated NFS server based on the Isis distributed system; contact Ken Birman or Keith Marzullo <ken@gvax.cs.cornell.edu>, <marzullo@gvax.cs.cornell.edu> for details. NFS has the advantage (over such protocols as ATT RFS) of being simple enough to make such a value-added server possible. However, this requires adding a substantial amount of state information to the NFS server, and allows much less inteligent replication than you could get if you had smart rfs clients that participated in the replication/ synchronization (implying, I think, that an rNFS must give poor performance, though Ken might disagree). Locus has the singular advantage of having been built from the ground up to allow file replication.