On Friday 16 May 2008, Andrew Morton wrote: > On Fri, 16 May 2008 20:20:30 -0400 Theodore Tso <tytso@xxxxxxx> wrote: > > On Fri, May 16, 2008 at 11:53:04PM +0100, Jamie Lokier wrote: > > > > > If you just want to test the block I/O layer and drive itself, > > > > > don't use the filesystem, but write a program which just access the > > > > > block device, continuously writing with/without barriers every so > > > > > often, and after power cycle read back to see what was and wasn't > > > > > written. > > > > > > > > Well, I think it is worth testing through the filesystem, different > > > > journaling mechanisms will probably react^wcorrupt in different ways. > > > > > > I agree, but intentional tests on the block device will show the > > > drives characteristcs on power failure much sooner and more > > > consistently. Then you can concentrate on the worst drivers :-) > > > > I suspect the real reason why we get away with it so much with ext3 is > > that the journal is usually contiguous on disk, hence, when you write > > to the journal, it's highly unlikely that commit block will be written > > and the blocks before the commit block have not. > > yup. Plus with a commit only happening once per few seconds, the time > window for a corrupting power outage is really really small, in > relative terms. All these improbabilities multiply. Well, the barriers happen like so (even if we actually only do one barrier in submit_bh, it turns into two) write log blocks flush #1 write commit block flush #2 write metadata blocks I'd agree with Ted, there's a fairly small chance of things get reordered around flush #1. flush #2 is likely to have lots of reordering though. It should be easy to create situations where the metadata for a transaction is written before the log blocks ever see the disk. EMC did a ton of automated testing around this when Jens and I did the initial barrier implementations, and they were able to trigger corruptions in fsync heavy workloads with randomized power offs. I'll dig up the workload they used. -chris -- 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
