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

[Phillip Susi]

Jens Axboe wrote:
> No Stephan is right, the barrier is both an ordering and integrity
> constraint. If a driver completes a barrier request before that request
> and previously submitted requests are on STABLE storage, then it
> violates that principle. Look at the code and the various ordering
> options.

I am saying that is the wrong thing to do.  Barrier should be about 
ordering only.  So long as the order they hit the media is maintained, 
the order the requests are completed in can change.  barrier.txt bears 
this out:

"Requests in ordered sequence are issued in order, but not required to 
finish in order.  Barrier implementation can handle out-of-order 
completion of ordered sequence.  IOW, the requests MUST be processed in 
order but the hardware/software completion paths are allowed to reorder 
completion notifications - eg. current SCSI midlayer doesn't preserve 
completion order during error handling."