Re: SAS v SATA interface performance

[Mark Lord <liml@rtr.ca>]

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