[Linux-cluster] EXT3 service mounted on two nodes

[Bryn M. Reeves]

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gordan at bobich.net wrote:
> Sure, that works. The only problem is that if both nodes write to the
> same files at the same time (including meta data), you'll end up with a
> corrupted file system. But if both machines are mounting the FS
> read-only (in which case you might as well use ext2), then there's no
> problem with that.

Are you sure about that? I've never known it do anything useful beyond
spectacularly breaking the file systems involved.

This is especially true for ext3 due to its journaling capability - as
soon as the two hosts start squabbling over the same journal block (i.e.
they detect an inconsistency caused by the other host overwriting
something) they're both going to abort the journal via an ext3_abort()
call and take the file system readonly.

In my experience, this tends to happen pretty quickly - the moment the
second host begins replaying the incomplete journal entries the first
host has outstanding.

Also, if mounting an ext3 volume read only you might want to consider
making the underlying block device read only too - this prevents any
problems with a host accidentally attempting journal recovery.

Regards,
Bryn.

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