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Re: how to prevent filesystem check

On Tue, Nov 28, 2006 at 09:20:26PM -0800, Andreas Dilger wrote:
> This is a good reason for the multi-mount protection feature that I
> proposed previously.  It would mark the filesystem as in-use on one 
> node and the filesystem itself would refuse to mount on the second
> node.  Unfortunately, this idea met resistance from some of the
> other ext3 developers from merging it upstream.

The resistance was because it means we have to put what is effectively
a cluster filesystem's distributed lock manager (DLM) just to tell
users that "News flash!  ext3 isn't a cluster filesystem" and then
error-out the mount.  Granted, it was a relatively simple cluster DLM,
but that's what you effectively need, complete with issues surrounding
heartbeats for liveness detection --- and since it was a simple
cluster DLM, it didn't handle temporary connectivity failure since
there was no STONITH (shoot-the-other-node-in-the-head) functionality.
So it didn't even solve the problem completely.

Still, if a lot of users are making this fundamental mistake of trying
to use ext3 as a cluster filesystem, maybe we need to revisit this
question, since hopefully once the user sees the error message they
won't keep doing this.  It doesn't stop them from wasting a lot of
time trying to set up such a system only discover that they used the
wrong tool in the first place, though.  So this feels more like a
documentation problem; but maybe it's worth it just as a backup to
some kind of documentation telling users that they really want to be
using OCFS2, GFS, GPFS, or some other cluster filesystem if they want
to do something like this.

					- Ted

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