Re: [RFC] relaxed barrier semantics

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 27, 2010 at 01:42:45PM -0500, James Bottomley wrote:
> I hope not ... I hope that if the drive reports write through or no
> cache that we don't enable (flush) barriers by default.

drivers/scsi/sd.c:sd_revalidate_disk()

	if (sdkp->WCE)
		ordered = sdkp->DPOFUA
			? QUEUE_ORDERED_DRAIN_FUA : QUEUE_ORDERED_DRAIN_FLUSH;
	else
		ordered = QUEUE_ORDERED_DRAIN;

	blk_queue_ordered(sdkp->disk->queue, ordered);

Documentation/block/barrier.txt:

QUEUE_ORDERED_DRAIN
        Requests are ordered by draining the request queue and cache
        flushing isn't needed.

        Sequence: drain => barrier


> The problem case is NV cache arrays (usually an array with a battery
> backed cache).  There's no consistency issue since the array will
> destage the cache on power fail but it reports a write back cache and we
> try to use barriers.  This is wrong because we don't need barriers for
> consistency and they really damage throughput.

The arrays I have access to (various Netapp, IBM and LSI) never report
write cache enabled.  I've only heard about the above issue from
historic tales.