Subject: Re: dual-ported raid

Jeff Garlough neotericgnosis at
Tue Oct 17 23:04:53 UTC 2006

>> What I would like to do is use one
>> normally, and mount the second system read-only to
perform backups and
>> to rsync the filesystem to another filesystem. When
it's mounted
>> read-write from another system, will mounting the
same filesystem
>> read-only cause the journal to be committed at the
time it's mounted?
>Yes, that is very bad.

Can you elaborate why mounting a filesystem read-only
is "dangerous"?

>> If so, is that a bad thing, that is, will it
corrupt the filesystem?
>Yes, it can corrupt the filesystem.

I assume, then, that mounting the filesystem read-only
flushes the
journal. Why does flushing it "early" corrupt the

>> Are journal events handled similar to databases,
with regard to transaction
>> processing of journal events, or could playing
"partial" journal events
>> (if there is such a thing) cause corruption?  Is
mounting the read-only
>> instance as a ext2 filesystem the best solution, or
does it matter if
>> it's mounted ext2 or ext3 as long as it's
>You can't mount it as ext2.

Why? It seemed to work, although I'm not sure, from
the comments I've
been getting, that it's safe. The ext3-faq says:
   How do I convert my ext3 partition back to ext2?

   Actually there is only little need to do so,
because in most cases it
   is sufficient to mount the partition explicitely as

>I would instead use a block-device level backup, like
"dump" if you really
>need to do it this way.  You are probably better off
just doing the backup
>from the primary node.
>Cheers, Andreas
>Andreas Dilger
>Principal Software Engineer
>Cluster File Systems, Inc.

Jeff Garlough

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