Hi! >>> Sure --- but name **any** filesystem that can deal with the fact that >>> 128k or 256k worth of data might disappear when you pull out the flash >>> card while it is writing a single sector? >> >> First... I consider myself quite competent in the os level, yet I did >> not realize what flash does and what that means for data >> integrity. That means we need some documentation, or maybe we should >> refuse to mount those devices r/w or something. >> >> Then to answer your question... ext2. You expect to run fsck after >> unclean shutdown, and you expect to have to solve some problems with >> it. So the way ext2 deals with the flash media actually matches what >> the user expects. (*) > > you loose data in ext2 Yes. >> OTOH in ext3 case you expect consistent filesystem after unplug; and >> you don't get that. > > the problem is that people have been preaching that journaling > filesystems eliminate all data loss for no cost (or at worst for minimal > cost). > > they don't, they never did. > > they address one specific problem (metadata inconsistancy), but they do > not address data loss, and never did (and for the most part the > filesystem developers never claimed to) Well, in case of flashcard and degraded MD Raid5, ext3 does _not_ address metadata inconsistency problem. And that's why I'm trying to fix the documentation. Current ext3 documentation says: #Journaling Block Device layer #----------------------------- #The Journaling Block Device layer (JBD) isn't ext3 specific. It was #designed #to add journaling capabilities to a block device. The ext3 filesystem #code #will inform the JBD of modifications it is performing (called a #transaction). #The journal supports the transactions start and stop, and in case of a #crash, #the journal can replay the transactions to quickly put the partition #back into #a consistent state. There's no mention that this does not work on flash cards and degraded MD Raid5 arrays. > people somehow have the expectation that ext3 does the data equivalent of > solving world hunger, it doesn't, it never did, and it never claimed > to. It claims so, above. > personally I don't consider the two filesystems to be significantly > different in terms of the data loss potential. I think people are more > aware of the potentials with XFS than with ext3, but I believe that the > risk of loss is really about the same (and pretty much for the same > reasons) Ack here. >> Again, ext2 handles that in a way user expects it. >> >> At least I was teached "ext2 needs fsck after powerfail; ext3 can >> handle powerfails just ok". > > you were teached wrong. the people making these claims for ext3 didn't > understand what ext3 does and doesn't do. Cool. So... can we fix the documentation? 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-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
