David Greaves <david@xxxxxxxxxxxx> wrote: > Peter T. Breuer wrote: > > >>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 - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
