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.
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