[Linux-cluster] EXT3 service mounted on two nodes

[gordan at bobich.net]

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

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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.

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