On Thu, May 31 2007, David Chinner wrote: > On Thu, May 31, 2007 at 08:26:45AM +0200, Jens Axboe wrote: > > On Thu, May 31 2007, David Chinner wrote: > > > IOWs, there are two parts to the problem: > > > > > > 1 - guaranteeing I/O ordering > > > 2 - guaranteeing blocks are on persistent storage. > > > > > > Right now, a single barrier I/O is used to provide both of these > > > guarantees. In most cases, all we really need to provide is 1); the > > > need for 2) is a much rarer condition but still needs to be > > > provided. > > > > > > > if I am understanding it correctly, the big win for barriers is that you > > > > do NOT have to stop and wait until the data is on persistant media before > > > > you can continue. > > > > > > Yes, if we define a barrier to only guarantee 1), then yes this > > > would be a big win (esp. for XFS). But that requires all filesystems > > > to handle sync writes differently, and sync_blockdev() needs to > > > call blkdev_issue_flush() as well.... > > > > > > So, what do we do here? Do we define a barrier I/O to only provide > > > ordering, or do we define it to also provide persistent storage > > > writeback? Whatever we decide, it needs to be documented.... > > > > The block layer already has a notion of the two types of barriers, with > > a very small amount of tweaking we could expose that. There's absolutely > > zero reason we can't easily support both types of barriers. > > That sounds like a good idea - we can leave the existing > WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED > behaviour that only guarantees ordering. The filesystem can then > choose which to use where appropriate.... Precisely. The current definition of barriers are what Chris and I came up with many years ago, when solving the problem for reiserfs originally. It is by no means the only feasible approach. I'll add a WRITE_ORDERED command to the #barrier branch, it already contains the empty-bio barrier support I posted yesterday (well a slightly modified and cleaned up version). -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
