Whoops, nevermind, it seems blkdev_issue_flush after commit does the barrier, I see it now. It's just rhel5 kernel that is affected. On Dec 11, 2009, at 1:45 AM, Oleg Drokin wrote: > Hello! > > I think ext4 ASYNC_COMMIT feature is potentially pretty unsafe > when write-back cache is enabled on the device. > Since no barriers are ever done with this feature even if > the barriers are enabled, we might end up in the situation > where we write the journal blocks, then commit block, they > hit the device write-back cache, after that actual metadata > blocks would be allowed to go to disk and eventually they will. > > In the end the device might decide to reorder some of the > actual metadata updates in front of journal updates and > if metadata updates will hit the disk and a power or other > failure occurs after that, we have inconsistent filesystem > as a result. > > I do not see an easy way to remedy the problem in this case > other than to insert empty barrier after the commit block > and wait for it completion, but I think that would negate > the entire gain from this feature. I wish we actually had > real ordered writes implemented, not just barrier/FUA > sent to the device before every ordered buffer. > > Am I missing something? > > Thanks. > > Bye, > Oleg -- 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
