Re: link() not increasing link count on NFS server

On Thu, 15 Nov 2007 02:42:48 -0500
Adam McDougall <mcdouga9_at_egr.msu.edu> wrote:

Moi,

> Hi, lately I've been trying to work out some NFS multiple access issues relating 
> to the Dovecot IMAP server software.  One symptom seems to be an unusual behavior 
> of FreeBSD NFS clients that I cannot reproduce with Linux or Solaris NFS clients.  
> Basically, Timo (cc'ed) came up with a small test case that seems to indicate 
> sometimes a link() call can succeed while the link count of the file will not 
> increase.

There is no foolproof way to do this over NFSv{2,3} using links,
symlinks or directories. In order to better deal with problems of this
sort NFSv3 changed the CREATE RPC to add support for an EXCLUSIVE flag
and a create verifier.

From RFC 1813, NFS Version 3 Protocol: "
   CREATE
         An exclusive flag and a create verifier was added for
         the exclusive creation of regular files.
"

The corresponding change was never made to any of the other RPC calls.

If possible you should change Dovecot to instead create lockfiles
using open(2) with the O_EXCL flag. This way your locks will work
reliably over NFSv3 even during server reboots or long lasting network
partitions. You will still have a problem if the underlying file
system is NFSv2 and perhaps also with other filesystems.

Cheers,
/b

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