Linux 2.4/2.5 SCSI considerably slower than FreeBSD

[Matthias Andree <matthias.andree@stud.uni-dortmund.de>]

Hello,

Marco Flohrer has posted an inquiry to de.comp.os.unix.linux.hardware
[German] <slrnae8q66.go4.marco.flohrer@diamond.csn.tu-chemnitz.de> that his
Seagate 36ES2 was slow with a DawiControl 2976UW (SYM53C875), only
around 25 MB/s. I have the same observation with a Fujitsu MAH3182MP
with an Adaptec 2940UW Pro which is not much faster. Either bus has an
active LVD/SE terminator.

Single-user mode,
time dd if=/dev/XXX of=/dev/null bs=65536 count=10240
(671,1 MB) linear read.

Table shows throughput in decimal MB/s (M = 1,000,000)

                               2.5  2.4  FBSD        max.
UWSCSI Fuj MAH3182MP  7200/min 32,1 29,4 35,1 TQ     40
UDMA66 Max 4W060H4    5400/min 27,1 26,7 25,7        66
UDMA66 IBM DTLA307045 7200/min 37,2 37,5 37,2 TQ 2.5 66
UDMA66 WDC AC420400D  5400/min 15,5 15,5 15,5 TQ 2.5 66
                               --------------
table is in decimal MB/s.

2.4:  Linux 2.4.19-pre2-ac3
2.5:  Linux 2.5.15
FBSD: FreeBSD 4.6-RC (Tagged Queueing Broken)

The IDE drives are attached to a VIA 82C686 (KT133), the Fujitsu
(actually an U-160 drive) to the mentioned Adaptec.

FBSD gets about 20% better throughput. It's far from perfect, but 90% of
the maximum is probably almost as good as we can get.

Why is Linux SCSI so slow?

-- 
Matthias Andree