On Fri, Apr 12, 2013 at 02:50:42PM +1000, Dave Chinner wrote: > > If that is the case, one possible solution that comes to mind would be > > to mark buffer_heads that contain metadata with a flag, so that the > > flusher thread can write them back at the same priority as reads. > > Ext4 is already using REQ_META for this purpose. We're using REQ_META | REQ_PRIO for reads, not writes. > I'm surprised that no-one has suggested "change the IO elevator" > yet..... Well, testing to see if the stalls go away with the noop schedule is a good thing to try just to validate the theory. The thing is, we do want to make ext4 work well with cfq, and prioritizing non-readahead read requests ahead of data writeback does make sense. The issue is with is that metadata writes going through the block device could in some cases effectively cause a priority inversion when what had previously been an asynchronous writeback starts blocking a foreground, user-visible process. At least, that's the theory; we should confirm that this is indeed what is causing the data stalls which Mel is reporting on HDD's before we start figuring out how to fix this problem. - Ted -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>