I think most of the information I have seen in this thread is
correct, but the circumstances for it being correct have
not been stated.
From my knowledge:
- if you use NFS version 3 AND you set the timeout
on the attribute cache to 0 (no attribute caching)
then there is no problem using NFS for anything
as long as you do your locking properly.
This is because NFSv3 has an atomic file create
operation which allows .lock files to work
ala UFS. Also, attribute information is returned
with every aknowledgement.
But, if you fail to get a fcntl lock, you must
close and re-open the file before trying to get the
lock again.
- if you use NFSv2 the .lock files are hard to make work
(lack of atomic create).
Philips comments on this are probably correct.
You can use a directory for locking because
NFSv3 does have an atomic mkdir operation.
- if your implementation of NFS on either client or
server is broken, then obviously all bets
are off. For a long time many clients and servers
had problems with fcntl locks, but I kind
of hope everybody has this working by now.
- if you use NFSv4 then the semantics of NFS are pretty much
identical to local disk (I think). You do not need
to do anything special (like play with the
attribute cache timeouts) to make things
work properly.
Does anybody with superior knowledge want to correct
any of the above?
Regards,
pdg
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