[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Bill Davidsen]

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 at tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979