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Re: Issues with network file systems and CVS

From: Frederic Brehm
Subject: Re: Issues with network file systems and CVS
Date: Tue, 17 Sep 2002 17:26:33 -0400

At 04:54 PM 9/17/2002, Larry Jones wrote:
Frederic Brehm writes:
> The technical reason for the failure has to do with locking in the
> repository. Locking is a tricky thing to do in a distributed system.
> Implementations often have subtle bugs.

A number of people have now said that and, while it may be true, it
doesn't apply to CVS.  CVS's locking scheme ensures that only one
process is writing to a repository directory at a time, even across
network filesystems.  The only known potential failure mode, which has
never been reported as actually occurring, results in *eveyone* being
locked out of a directory, not allowing two servers to write at the same

But, isn't a network filesystem a distributed system? I wasn't referring to CVS by itself as a distributed system with the bug. It's the distributed filesystem that CVS uses where the bugs are.

No, the problem is just that network filesystems, particularly those
that run over unreliable connections like NFS traditionally does, are
extremely tricky to get right.  They're even trickier when you don't
control both ends of the connection, as when the client software is
supplied by a different vendor than the server software.  All the
problems I've seen seem to be either handshaking bugs -- where the
client and the server don't communicate reliably -- or simple
housekeeping bugs in the server -- where data intended for the disk
never actually gets there.  Why CVS should be so good at triggering
these problems, I don't know.  I do know that the commercial CAD/CAM/CAE
system my employer produces is also good at triggering them, though.

Yup. We agree. It's the filesystem.

I suppose that we could get into a semantic argument where we quibble over whether CVS+NFS is an example of a distributed system. But, let's not go there.


Frederic W. Brehm, Sarnoff Corporation,

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