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

[Jens Axboe <jens.axboe@oracle.com>]

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).

-- 
Jens Axboe

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