Re: ext3 journal on software raid (was Re: PROBLEM: Kernel 2.6.10 crashing repeatedly and hard)

David Greaves [off-list ref] wrote:

Peter T. Breuer wrote:

quoted

quoted

ext3 journals are much safer on mirrored devices than on non-mirrored

That's irrelevant - you don't care what's in the journal, because if
your system crashes before committal you WANT the data in the journal
to be lost, rolled back, whatever, and you don't want your machine to
have acked the write until it actually has gone to disk.

Or at least that's what *I* want. But then everyone has different
wants and needs. What is obvious, however, are the issues involved.

If the journal is safely written to the journal device and the machine 

You don't know it has been. Raid can't tell.

crashes whilst updating the main filesystem you want the journal to be 
replayed, not erased. The journal entries are designed to be replayable 
to a partially updated filesystem.

It doesn't work. You can easily get a block  written to the journal on
disk A, but not on disk B (supposing raid 1 with disks A and B).
According to you "this" should be replayed. Well, which result do you
want? Raid has no way of telling.

Suppose that A contains the last block to be written to a file, and
does not. Yet B is chosen by raid as the "reliable" source.

Then what happens? 

Is the transaction declared "completed" with incomplete data? With
incorrect data?

Myself I'd hope it were rolled back, whichever of A or B were chosen,
because some final annotation was missing from the journal, saying
"finished and ready to send" (alternating bit protocol :-). But you
can't win ... what if the "final" annotation were written to journal on
A but not on B.

Then what would happen?

Well, then whichever of A or B the raid chose, you'd either get the
data rolled forward or backward.


Which would you prefer? 

I'd just prefer that it was all rolled back. 

That's the whole point of journalling filesystems, write the deltas to 
the journal, make the changes to the fs, delete the deltas from the journal.

Consider the above. There is no magic.

If the machine crashes whilst the deltas are being written then you 
won't play them back - but your fs will be consistent.

What if the delta is written to one journal, but not to the other, when
the machine crashes?

I outlined the problem above. You can't win this game.

Peter