EXT3 on raid with external journal...

Kurt Fitzner kfitzner at excelcia.org
Fri Apr 16 14:20:13 UTC 2004

Matt Bernstein wrote:
> On Apr 13 Kurt Fitzner wrote:
> There could be metadata which is only in the journal, so failure probably 
> means reboot + full fsck, so you may as well use ext2 if your machine 
> doesn't otherwise crash.
> Far preferable, I think, would be to put your journal on a RAID 1 pair.

I would like to think that if the ext3 driver encountered an error 
writing to the journal, that it would then skip the journal and write 
straight to the device - reverting to ext2 behavior.  There should never 
be any loss of data (meta or otherwise) upon the failure of a journal 
device.  That is, unless the failure of the journaling device coincides 
with a power failure.  That is:

1) Failure of journaling device
2) Attempted write of metadata to journal device
3) Power failure before ext3 gives up on the journaling device

In that scenario, the ramification is the array requiring a full fsck. 
The benefit of running the journal on an external device would far 
outweigh the cost of a full fsck in the unlikely event the above happens.

I need to know, though, what exactly is the behavior of ext3 in the 
following situations:
  - At system startup if there is a failure to "mount" an external journal
  - During operation if the external journal device fails.

Does ext3 then revert to non-journaled (ext2) behavior in those instances?


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