(dunny why you explicitly dropped me off the cc/to list when replying to my email, hence I missed it for 3 days) On Fri, May 25 2007, Phillip Susi wrote: > Jens Axboe wrote: > >A barrier write will include a flush, but it may also use the FUA bit to > >ensure data is on platter. So the only situation where a fallback from a > >barrier to flush would be valid, is if the device lied and told you it > >could do FUA but it could not and that is the reason why the barrier > >write failed. If that is the case, the block layer should stop using FUA > >and fallback to flush-write-flush. And if it does that, then there's > >never a valid reason to switch from using barrier writes to > >blkdev_issue_flush() since both methods would either both work or both > >fail. > > IIRC, the FUA bit only forces THAT request to hit the platter before it > is completed; it does not flush any previous requests still sitting in > the write back queue. Because all io before the barrier must be on the > platter as well, setting the FUA bit on the barrier request means you > don't have to follow it with a flush, but you still have to precede it > with a flush. I'm well aware of how FUA works, hence the barrier FUA implementation does flush and then write-fua. The win compared to flush-write-flush is just a saved command, essentially. > >It's not block layer breakage, it's a device issue. > > How isn't it block layer breakage? If the device does not support > barriers, isn't it the job of the block layer ( probably the scheduler ) > to fall back to flush-write-flush? The problem is flush not working, the block layer can't fix that for you obviously. If it's FUA not working, the block layer should fall back to flush-write-flush, as they are obviously functionally equivalent. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
