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Message-ID: <4661CB1C.60806@tmr.com> Date: Sat, 02 Jun 2007 15:55:08 -0400 From: Bill Davidsen <davidsen@....com> To: Jens Axboe <jens.axboe@...cle.com> CC: David Chinner <dgc@....com>, david@...g.hm, Phillip Susi <psusi@....rr.com>, Neil Brown <neilb@...e.de>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, dm-devel@...hat.com, linux-raid@...r.kernel.org, Stefan Bader <Stefan.Bader@...ibm.com>, Andreas Dilger <adilger@...sterfs.com>, Tejun Heo <htejun@...il.com> Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. Jens Axboe wrote: > On Fri, Jun 01 2007, Bill Davidsen wrote: > >> Jens Axboe wrote: >> >>> On Thu, May 31 2007, Bill Davidsen wrote: >>> >>> >>>> 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? >>>> >>>> >>> See the above stuff you quote, it's answered there. It's not a change, >>> this is how the Linux barrier write has always worked since I first >>> implemented it. What David and I are talking about is adding a more >>> relaxed version as well, that just implies ordering. >>> >>> >> I was reading the documentation in block/biodoc.txt, which seems to just >> say ordered: >> >> 1.2.1 I/O Barriers >> >> There is a way to enforce strict ordering for i/os through barriers. >> All requests before a barrier point must be serviced before the barrier >> request and any other requests arriving after the barrier will not be >> serviced until after the barrier has completed. This is useful for >> higher >> level control on write ordering, e.g flushing a log of committed updates >> to disk before the corresponding updates themselves. >> >> A flag in the bio structure, BIO_BARRIER is used to identify a >> barrier i/o. >> The generic i/o scheduler would make sure that it places the barrier >> request and >> all other requests coming after it after all the previous requests >> in the >> queue. Barriers may be implemented in different ways depending on the >> driver. A SCSI driver for example could make use of ordered tags to >> preserve the necessary ordering with a lower impact on throughput. >> For IDE >> this might be two sync cache flush: a pre and post flush when >> encountering >> a barrier write. >> >> The "flush" comment is associated with IDE, so it wasn't clear that the >> device cache is always cleared to force the data to the platter. >> > > The above should mention that the ordered tag comment for SCSI assumes > that the drive uses write through caching. If it does, then an ordered > tag is enough. If it doesn't, then you need a bit more than that (a post > flush, after the ordered tag has completed). > > Thanks, go it. -- bill davidsen <davidsen@....com> CTO TMR Associates, Inc Doing interesting things with small computers since 1979 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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