Hi! >> I can reproduce data loss with ext3 on flashcard in about 40 >> seconds. I'd not call that "odd event". It would be nice to handle >> that, but that is hard. So ... can we at least get that documented >> please? >> > > Part of documenting best practices is to put down very specific things > that do/don't work. What I worry about is producing too much detail to > be of use for real end users. Well, I was trying to write for kernel audience. Someone can turn that into nice end-user manual. > I have to admit that I have not paid enough attention to this specifics > of your ext3 + flash card issue - is it the ftl stuff doing out of order > IO's? The problem is that flash cards destroy whole erase block on unplug, and ext3 can't cope with that. >> _All_ flash cards (MMC, USB, SD) had the problems. You don't need to >> get clear grasp on trends. Those cards just don't meet ext3 >> expectations, and if you pull them, you get data loss. >> > Pull them even after an unmount, or pull them hot? Pull them hot. [Some people try -osync to avoid data loss on flash cards... that will not do the trick. Flashcard will still kill the eraseblock.] >>> Nothing is perfect. It is still a trade off between storage >>> utilization (how much storage we give users for say 5 2TB drives), >>> performance and costs (throw away any disks over 2 years old?). >>> >> >> "Nothing is perfect"?! That's design decision/problem in raid5/ext3. I >> believe that should be at least documented. (And understand why ZFS is >> interesting thing). >> > Your statement is overly broad - ext3 on a commercial RAID array that > does RAID5 or RAID6, etc has no issues that I know of. If your commercial RAID array is battery backed, maybe. But I was talking Linux MD here. >> And I still use my zaurus with crappy DRAM. >> >> I would not trust raid5 array with my data, for multiple >> reasons. The fact that degraded raid5 breaks ext3 assumptions should >> really be documented. > > Again, you say RAID5 without enough specifics. Are you pointing just at > MD RAID5 on S-ATA? Hardware RAID cards? A specific commercial RAID5 > vendor? Degraded MD RAID5 on anything, including SATA, and including hypothetical "perfect disk". >> The papers show failures in "once a year" range. I have "twice a >> minute" failure scenario with flashdisks. >> >> Not sure how often "degraded raid5 breaks ext3 atomicity" would bite, >> but I bet it would be on "once a day" scale. >> >> We should document those. > > Documentation is fine with sufficient, hard data.... Degraded MD RAID5 does not work by design; whole stripe will be damaged on powerfail or reset or kernel bug, and ext3 can not cope with that kind of damage. [I don't see why statistics should be neccessary for that; the same way we don't need statistics to see that ext2 needs fsck after powerfail.] 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
