Re: [RFC] ext3 multi-mount protection

On Jul 14, 2006  18:45 +0100, Stephen C. Tweedie wrote:
> On Fri, 2006-07-14 at 08:26 -0600, Andreas Dilger wrote:
> tune2fs is generally safe on a *locally* mounted fs.  Remotely mounted
> is a different matter altogether; tune2fs will end up doing a non-atomic
> read-modify-write which can interfere with superblock writes on the
> other node.  It's not likely to be a long-lasting problem though, as the
> next sb write will restore things; but even then it does mean that the
> tune2fs update will be lost unexpectedly.

Ah, true, because the disk copy doesn't necessarily match the in-memory
copy on the remote node.

> e2fsck is worse because it not only needs to be able to test that the fs
> isn't mounted, it also needs to mark the fs as in-use while it's
> operating, and that requires it to be running the sequence-kicker in the
> background.

Or it could have a magic "e2fsck" override (with hostname) for runs that
don't have "-n" that doesn't need to be refreshed, and either e2fsck
completes successfully or the user gets a clear message that e2fsck is
running on $hostname and either wait until it is finished, or the admin
must manually run "tune2fs -e2fsck-is-no-longer-running /dev/foo".

> Something like the exclusive-open flag in cLVM sounds like the right
> sort of layer to have this at.  But yes, there's no denying that a
> cluster infrastructure is something heavyweight enough that it would be
> nice to be able to avoid it.

And the fact that not everything is running cLVM (e.g. FC shared LUNs
on external RAID systems) and there is no guarantee that cLVM is any
better configured than HA/CluManager.

It should be clearly stated that this functionality is only needed and
enabled on systems that might be running in an HA environment, or if
they are running on a SAN which isn't filtering LUNs on a per-node basis.
There will be no impact/overhead if it isn't enabled.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.



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