Re: journal on an ssd

[Andreas Dilger <adilger sun com>]

On Sep 11, 2008  07:43 +0200, Tobias Oetiker wrote:
> You are telling me things that I am aware of. The reason I wrote to
> this group is to figure what would happen to an ext3 fs when the
> external journal was lost, especially what happens when it is lost
> on a filesystem where 'data=journal' is set.

Losing a journal will, in 99% of the cases, mean the loss of only a
few seconds of data.  In some rare cases it may be that an inconsistency
from a partially-updated commit will cause e2fsck to become confused
and possibly clean up a small number more files than it would have
otherwise.

> Because if it is catastrophic, then it basically means that the
> journal has to reside on a device that is as secure as to rest of
> the data, meaning that if the data is on RAID6 then the journal
> should be on RAID6 too.

No, because RAID6 is terribly sucky for performance.  If you need this
kind of reliability triple-mirrored RAID 1 would be better.  Much less
CPU overhead, and no extra IO.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.