Michael Evans wrote:
So, you're saying that the MD layer is doing it's own buffering? Are
you sure? With the system cache disabled and the drive (and block
device driver) cache disabled, there should be no reason to require
synchronous I/O, unless, as you suggest, the MD layer is broken.
You're saying that both O_DIRECT and O_SYNC must be used to disable
cache effects. Why then are there two separate flags and not just one?
Synchronous is a very different behavior that is not necessary for
this test and put additional requirements that are not needed for this
test.
Chris
Reading the manual page it seems O_DIRECT explicitly minimizes any
attempts at extra copying; not explicitly disabling buffers, merely
not adding more. In another mail thread this tweak-able was
discussed:
echo 0 > /sys/block/md*/md/stripe_cache_size
stripe_cache_size only exists for raid4, raid5 and raid6.
It never existed for other raid levels because it's pointless.
/mjt
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