Am Mittwoch, 25. November 2015, 07:32:34 CET schrieb Austin S Hemmelgarn: > On 2015-11-24 17:26, Eric Sandeen wrote: > > On 11/24/15 2:38 PM, Austin S Hemmelgarn wrote: > >> if the system was > >> shut down cleanly, you're fine barring software bugs, but if it > >> crashed, you should be running a check on the FS. > > > > Um, no... > > > > The *entire point* of having a journaling filesystem is that after a > > crash or power loss, a journal replay on next mount will bring the > > metadata into a consistent state. > > OK, first, that was in reference to BTRFS, not ext4, and BTRFS is a COW > filesystem, not a journaling one, which is an important distinction as > mentioned by Hugo in his reply. Second, there are two reasons that you > should be running a check even of a journaled filesystem when the system > crashes (this also applies to COW filesystems, and anything else that > relies on atomicity of write operations for consistency): > > 1. Disks don't atomically write anything bigger than a sector, and may > not even atomically write the sector itself. This means that it's > possible to get a partial write to the journal, which in turn has > significant potential to put the metadata in an inconsistent state when > the journal gets replayed (IIRC, ext4 has a journal_checksum mount > option that is supposed to mitigate this possibility). This sounds like > something that shouldn't happen all that often, but on a busy > filesystem, the probability is exactly proportionate to the size of the > journal relative to the size of the FS. > > 2. If the system crashed, all code running on it immediately before the > crash is instantly suspect, and you have no way to know for certain that > something didn't cause random garbage to be written to the disk. On top > of this, hardware is potentially suspect, and when your hardware is > misbehaving, then all bets as to consistency are immediately off.
In the case of shaky hardware a fsck run can report bogus data, i.e. problems where they are none or vice versa. If I suspect defect memory or controller I would check the device on different hardware only. Especially on attempts to repair any possible issues. -- Martin -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
