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Re: [Linux-cluster] EXT3 service mounted on two nodes



On Mon, 29 Oct 2007, Bryn M. Reeves wrote:

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gordan 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.

I can't see why it would. If the fs is mounted ro, no harm will come to it. Any writing, though, and you're likely to trash it faster than you can type mount -o ro,remount. :-)

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.

As I said, any writes (including meta-data - which includes journals), and the fs will be destroyed pretty quickly.

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.

Yup, that sounds about right. :-)

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.

Sure - you're probably better of mounting it as ext2, which I also mentioned.

Gordan


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