the performance of SAS is not better than SCSI in FreeBSD, WHY?

Ivan Voras ivoras at
Tue Apr 10 21:05:08 UTC 2007

Tzu-Hua Wang wrote:

> Generally, the performace of SAS in FreeBSD is far worse than SCSI in
> FreeBSD.
> Is it normal? How can I do to improve the performance of SAS HD in FreeBSD?

There's a huge number of possible reasons - unless you're testing the
same controller (I think there exist SAS+SCSI controllers, I might be
wrong), you can't conclude that the SAS interface is less supported.

For what it's worth, I've got approximately the same numbers on DL380
G5, but I've attributed them to 2.5" small form factor drives.

2x 2.5" 10k RPM, CISS hw RAID1		2x 3.5", 7.5k RPM, soft RAID1
Full stroke		3.863 ms	5.942 ms
Half stroke		3.929 ms	6.186 ms
Quarter stroke		3.998 ms	5.700 ms
Short forward		3.889 ms	4.060 ms
Short backward		4.715 ms	4.748 ms
Seqential outer		1.011 ms	0.167 ms
Sequential inner	1.013 ms	0.177 ms
Read transfer, outside	43.6 MB/s	61.2 MB/s
Read transfer, middle	38.8 MB/s	54.8 MB/s
Read transfer, inside	32.8 MB/s	34.9 MB/s

The 3.5" drives were SATA in a different machine (Dell's). It might be
that the CISS driver is unreasonably slow, which is kind of possible
given the suspicious lower bound of about 1 ms in seeks. Maybe something
is throttling the I/O rate?

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