How Make a File-Replica Server?

[Larry Carroll]

We have an Ethernet network of a bunch of DECstation 3100s running X & 
Motif.  Currently the X is DEC's DECwindows, which is X11R3 (or so I'm 
told), but we're soon migrating to a generic X11R4.  There's also a 
VAX with about a gigabyte of disc space that we'll use to store raster
maps.  All systems run Ultrix & are NFS'd together.

We're writing a map-viewing & -annotating program which will run on the
workstations.  The VAX will act mostly as a file server.  For reason I don't
fully understand, the designers wants to allow users to launch as many
copies of the map program as they choose to.  They also want to use a local
server to maintain working replicas of map files on the local hard disc for
the map program.  This local server will run as a daemon & (presumably) by 
started up each time the system is booted.

My question is: what's the best way implement this daemon?  Where can I read
up on the trade-offs of various methods of implementing it?  Or does NFS have
some way of automatically maintaining these replicas the way the NCS system
does?  (At least the book we have on NCS suggests that, although I've not
been able to find any specifics on that.)  Can NFS be configured so that
other disks on the network act as caches, and if so can the cache contents be
made long-term?  (Users will often work on a particular map over a period of
days or even weeks.)