On May 29, 2007, at 1:25 PM, Lida Horn wrote:
Point one, the comments that Eric made do not give the complete
picture.
All the tests that Eric's referring to were done through ZFS
filesystem.
When sequential I/O is done to the disk directly there is no
performance
degradation at all.
Doing what test exactly? single stream from a single disk? What
type of disk are you using? My blog explicitly shows that there's no
difference with NCQ enabled or disabled when its just a single stream
sequential read (using the Hitachi AJOA disk).
Second point, it does not take any additional
time in ldi_strategy(), bdev_strategy(), mv_rw_dma_start(). In some
instance it actually takes less time. The only thing that sometimes
takes additional time is waiting for the disk I/O.
Right, this shows that most likely the disk (firmware) is doing
pointless work trying to re-order I/Os that we're already sequential.
always a pleasure lida,
eric
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