On Sun 2009-01-04 13:49:49, Rob Landley wrote: > On Saturday 03 January 2009 06:38:15 Pavel Machek wrote: > > +Ext3 expects disk/storage subsystem to behave sanely. On sanely > > +behaving disk subsystem, data that have been successfully synced will > > +stay on the disk. Sane means: > > + > > +* writes to media never fail. Even if disk returns error condition during > > + write, ext3 can't handle that correctly, because success on fsync was > > already + returned when data hit the journal. > > + > > + (Fortunately writes failing are very uncommon on disks, as they > > + have spare sectors they use when write fails.) > > + > > +* either whole sector is correctly written or nothing is written during > > + powerfail. > > + > > + (Unfortuantely, none of the cheap USB/SD flash cards I seen do behave > > + like this, and are unsuitable for ext3. > > Want to document the granularity issues with flash, while you're at it? > > An inherent problem with using flash as a normal block device is that the > flash erase size is bigger than most filesystem sector sizes. So when you > request a write, it may erase and rewrite the next 64k, 128k, or even a couple > megabytes on the really _big_ ones. > > If you lose power in the middle of that, ext3 won't notice that data in the > "sectors" _after_ the one your were trying to write to got trashed. > > The flash filesystems take this into account as part of their wear levelling > stuff (they normally copy the entire chunk into a new chunk, leaving the old > one in place until it's no longer needed), but they need to query the device > to get the erase granularity in order to do that, which is why they don't work > on non-flash block devices. Is there linux filesystem that can handle that? I know jffs2, but that's unsuitable for stuff like USB thumb drives, right? Does this sound like a fair summary? Sector writes are atomic (ATOMIC-SECTORS) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Either whole sector is correctly written or nothing is written during powerfail. Unfortuantely, none of the cheap USB/SD flash cards I seen do behave like this, and are unsuitable for all linux filesystems I know. An inherent problem with using flash as a normal block device is that the flash erase size is bigger than most filesystem sector sizes. So when you request a write, it may erase and rewrite the next 64k, 128k, or even a couple megabytes on the really _big_ ones. If you lose power in the middle of that, filesystem won't notice that data in the "sectors" _after_ the one your were trying to write to got trashed. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html