Re: [HACKERS] PG on NFS may be just a bad idea

Based on this analysis, I have added an NFS section to the tablespaces
portion of the documentation, and linked to it from 'Creating a database
cluster'. Patch attached.

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Tom Lane wrote:
> I spent a bit of time tonight poking at the issue reported here:
> http://archives.postgresql.org/pgsql-novice/2007-08/msg00123.php
>
> It turns out to be quite easy to reproduce, at least for me: start CVS
> HEAD on an NFS-mounted $PGDATA directory, and run the contrib regression
> tests ("make installcheck" in contrib/). I see more than half of the
> DROP DATABASE commands complaining in exactly the way Miya describes.
> This failure rate might be an artifact of the particular environment
> (I tested NFS client = Fedora Core 6, server = HPUX 10.20 on a much
> slower machine) but the problem is clearly real.
>
> In the earlier thread I cited suggestions that this behavior comes from
> client programs holding files open longer than they should. However,
> strace'ing this behavior shows no evidence at all that that is happening
> in Postgres. I have an strace that shows conclusively that the bgwriter
> never opened any file in the target database at all, and all earlier
> backends exited before the one doing the DROP DATABASE began its dirty
> work, and yet:
>
> [pid 19211] 22:50:30.517077 rmdir("base/18193") = -1 ENOTEMPTY (Directory not empty)
> [pid 19211] 22:50:30.517863 write(2, "WARNING: could not remove file "..., 79WARNING: could not remove file or directory "base/18193": Directory not empty
> ) = 79
> [pid 19211] 22:50:30.517974 sendto(7, "N\0\0\0rSWARNING\0C01000\0Mcould not "..., 115, 0, NULL, 0) = 115
>
> After some googling I think that the damage may actually be getting done
> at the kernel level. According to
> http://www.time-travellers.org/shane/papers/NFS_considered_harmful.html
> it is fairly common for NFS clients to cache writes, meaning that the
> kernel itself may be holding an old write and not sending it to the NFS
> server until after the file deletion command has been sent.
>
> (I don't have the network-fu needed to prove that this is happening by
> sniffing the network traffic; anyone want to try?)
>
> If this is what's happening I'd claim it is a kernel bug, but seeing
> that I see it on FC6 and Miya sees it on Solaris 10, it would be a bug
> widespread enough that we'd not be likely to get it killed off soon.
>
> Maybe we need to actively discourage people from running Postgres
> against NFS-mounted data directories. Shane Kerr's paper cited above
> mentions some other rather scary properties, including O_EXCL file
> creation not really working properly.
>
> regards, tom lane
>
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--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +