In article <[EMAIL PROTECTED]>, Lars Wirzenius <[EMAIL PROTECTED]> wrote:
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>[ Please don't Cc: public replies to me. ]
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>Karl M. Hegbloom:
>> What happens? Can you describe the problem? Explain your setup in
>> more detail, please. I would like to know more about the problems
>> that are encountered with nfs.
>
>When you try to create a lock file with open(2) using O_CREAT|O_EXCL
>(i.e., you ask the kernel to create the file, but fail if it exists
>already), the operation will sometimes fail, when you network drops
>a packet. That operation simply isn't guaranteed to work over NFS,
>even though it is for normal Unix filesystems.
It's more the case that the O_EXCL flag doesn't exist in the NFS protocol.
So the operation is not atomic. The kernel itself does:
[pseudo code]
if (flags & O_EXCL) {
if (stat(file) >= 0)
return EEXIST;
}
return open (file, flags);
This is atomic on local file systems but not on NFS file systems. Usually
it will work, but I tested it by running a program repeatedly getting
a lockfile on 2 computers on a NFS mounted file system, and every once
in a while this method fails, as you would expect ofcourse.
I once wrote a patch where the kernel does:
if (flags & O_EXCL) {
/*
* Here the kernel uses a method with link() ing to the
* file, similar to NFS mailbox locking, to create the
* file atomically
*/
...
...
}
return open (file, flags);
It worked like a charm, but Linus wouldn't accept it in the kernel.
Too bad, all of our problems would have been solved..
I now have open() in a preloaded library, /lib/nfslock.so that gets
preloaded on all our machines through /etc/ld.so.preload. Does about
the same thing, and lets us safely share mail over NFS. A bit (a bit?!)
of a hack, though.
Mike.
--
| Miquel van Smoorenburg | "I need more space" "Well, why not move to Texas" |
| [EMAIL PROTECTED] | "No, on my account, stupid." "Stupid? Uh-oh.." |
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