ext3 journal on software raid

Stephen C. Tweedie sct at redhat.com
Sat Jan 1 22:19:23 UTC 2005


On Fri, 2004-12-31 at 02:49, Andy Smith wrote:

> Are the comments made in this posting from the linux-raid list
> correct?

> http://marc.theaimsgroup.com/?l=linux-raid&m=110444288429682&w=2

No.  :-)  Some of the facts are.  The conclusions are not.

> There is nothing that attempts expliciitly to maintain the ordering in
> RAID (talking about mirroring here).

Disks and IO subsystems in general don't preserve IO ordering.  ext3 is
designed not to care.  As long as the raid device tells the truth about
when the data is actually committed to disk (all of the mirror volumes
are uptodate) for a given IO, ext3 should be quite happy.

> What's wrong is that the journal will be mirrored (if it's a mirror).
> That means that (1) its data will be written twice, which is a big deal
> since ALL the i/o goes through the journal first

Not true; by default, only metadata goes through the journal, not data.

> and (2) the journal
> is likely to be inconsistent (since it is so active) if you get one of
> those creeping invisible RAID corruptions that can crop up inevitably
> in RAID normal use.

Umm, if soft raid is expected to have silent invisible corruptions in
normal use, then you shouldn't be using it, period.  That's got zero to
do with journaling.


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