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. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
