How Make a File-Replica Server?

[Ian Hogg]

In article <1991Apr30.043758.2072 at jpl-devvax.jpl.nasa.gov> larryc at puente.jpl.nasa.gov (Larry Carroll) writes:
>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.)

   I think you are confused about NFS and NCS.  NFS is a network file system
as you know.  NCS is not a network file system.  It stands for Network Computing
System and is a type of RPC technology.  NFS machines communicate with each 
using Sun RPC's.  The replication available in NCS is called Data Replication
Manager (DRM) and only guarantees that all servers of a particular class will
eventually see all the calls made to the class of servers.  I believe that it is
not fully implemented.  It is only implemented in the Location Broker.

  Some file systems (like AFS) support file replication.  Your daemon 
implementation depends on how the application uses the map files.  If only one
user can access the file in write mode, why not just keep a list of checked
out files on the VAX and have each node copy the file to their local machine
when needed?


-- 
Ian Hogg                        email:  rathe!ian at cs.umn.edu
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