Messages in this thread | | | Date | Thu, 31 May 2007 09:30:03 -0400 | From | Bill Davidsen <> | Subject | Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. |
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Jens Axboe wrote: > 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). > > Wait. Do filesystems expect (depend on) anything but ordering now? Does md? Having users of barriers as they currently behave suddenly getting SYNC behavior where they expect ORDERED is likely to have a negative effect on performance. Or do I misread what is actually guaranteed by WRITE_BARRIER now, and a flush is currently happening in all cases?
And will this also be available to user space f/s, since I just proposed a project which uses one? :-( I think the goal is good, more choice is almost always better choice, I just want to be sure there won't be big disk performance regressions.
-- bill davidsen <davidsen@tmr.com> CTO TMR Associates, Inc Doing interesting things with small computers since 1979
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