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



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."



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