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[dm-devel] 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 tmr com>
 CTO TMR Associates, Inc
 Doing interesting things with small computers since 1979


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