YP required with NFS? (Really, redundant file servers)

[roy at phri.UUCP]

In article <59000005 at gorgo.UUCP> bsteve at gorgo.UUCP writes:
> Lets make them go away and fully abstract the mapping of files across the
> network as well as incorporate primary and secondary server hosts for groups
> of clients, to make the network more resistant to server crashes.

	How hard would it be to incorporate a "shadow server" mode into
NFS?  Imagine 2 servers serving the same file system.  When a write request
comes in, both servers do it and the client waits to hear an ack from both
of them.  When a read request comes in, both servers try to do it and the
client takes the data from whichever server responds first.  When one
server goes down, the other one just pick up all the read requests and the
write requests only get acked once (there has to be some way for the client
to know it's only waiting for a single ack; this is a sticky point).

	There is nothing new here; schemes like this have been implemented
many times before (I think DEC's HSC-50 can do this, for example).
Usually, though, it's done at the physical disk level, not the file system
level.  What kinds of problems would you run into if the two copies of the
file system had differing inode numbers, for example?  What happens when
these two supposedly identical file systems are almost full, but because of
slightly different fragmentation, one gets full before the other?  What
other horrible things might make this idea not work?
-- 
Roy Smith, {allegra,cmcl2,philabs}!phri!roy
System Administrator, Public Health Research Institute
455 First Avenue, New York, NY 10016

"you can't spell deoxyribonucleic without unix!"