>The "disk motor as a generator" tale may not be purely folklore. When >an IDE drive is not in writeback mode, something special needs to done >to ensure the last write to media is not a scribble.
No it doesn't. The last write _is_ a scribble. Systems that make atomic updates to disk drives use a shadow update mechanism and write the master sector twice. If the power fails in the middle of writing one, it will almost certainly be unreadable due to a CRC failure, and the other one will have either the old or new master block contents. And I think there's a problem with drives that, upon sensing the unreadable sector, assign an alternate even though the sector is fine, and you eventually run out of spares. Incidentally, while this primitive behavior applies to IDE (ATA et al) drives, that isn't the only thing people put filesystem on. Many important filesystems go on higher level storage subsystems that contain IDE drives and cache memory and batteries. A device like this _does_ make sure that all data that it says has been written is actually retrievable even if there's a subsequent power outage, even while giving the performance of writeback caching. -- Bryan Henderson IBM Almaden Research Center San Jose CA Filesystems -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
