Jens Axboe wrote:
On Mon, Dec 10 2007, Tejun Heo wrote:
There's one thing we can do to improve the situation tho. Several
drives including raptors and 7200.11s suffer serious performance hit if
sequential transfer is performed by multiple NCQ commands. My 7200.11
can do > 100MB/s if non-NCQ command is used or only upto two NCQ
commands are issued; however, if all 31 (maximum currently supported by
libata) are used, the transfer rate drops to miserable 70MB/s.
It seems that what we need to do is not issuing too many commands to one
sequential stream. In fact, there isn't much to gain by issuing more
than two commands to one sequential stream.
Well... CFQ wont go to deep queue depths across processes if they are
doing streaming IO, but it wont stop a single process from doing so. I'd
like to know what real life process would issue a streaming IO in some
async manner as to get 31 pending commands sequentially? Not very likely
..
In the case of the WD Raptors, their firmware has changed slightly over
the years. The ones I had here would *disable* internal read-ahead
for TCQ/NCQ commands, effectively killing any hope of sequential throughput
even for a queuesize of "1". This was acknowledged by people with inside
knowledge of the firmware at the time.
Cheers
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