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SubjectRe: slow scsi disk perf (sym53c875 and IBM drives)
> I've been having relatively poor scsi disk performance 
> for as long as I can remember now. I've experienced this
> with ~2.2.12 all the way up to 2.4.17.
>
> I'm not much of a scsi buff but I've read some
> suggestions such as proper termination, queue depths,
> etc but nothing seems to have helped. My understanding
> is that I should be getting better performance than I'm
> getting with these drives.

I have a few DCHS UW disks and get about 10 MB/sec on
a streaming read from them (slower toward the end of
the disk). You can make your own measurements bypassing
the block subsystem (and sd) with the sg_dd utility
found in sg3_utils package. That package is now listed
on freshmeat.net .

For example:
$ time sg_dd if=/dev/sg0 of=/dev/null bs=512 count=2M
to time a 1GB (approx) read at the start of the disk.
[Use sg_map to work out the mapping between the "sd"
and "sg" device names.]

If there is a major discrepancy between this figure
and what you are seeing from hdparm then there is
a problem.


On the disk in this box (Seagate ST318451LW 15Krpm)
I get:
hdparm -t /dev/sda

/dev/sda:
Timing buffered disk reads: 64 MB in 1.99 seconds = 32.16 MB/sec

In my sg_dd tests I get better than 40 MB/sec. The hdparm
test definitely was timing extraneous activity because
the disk light was not on all the time.

> Any suggestions would be appreciated and if this is not
> the right forum to ask.. please direct me to the proper
> place.

linux-scsi@vger.kernel.org is arguably a more appropriate
newsgroup.

Doug Gilbert



Rest of original post [for the linux-scsi list]
vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
hdparm output:

/dev/sda:
Timing buffered disk reads: 64 MB in 10.94 seconds = 5.85 MB/sec

/dev/sdb:
Timing buffered disk reads: 64 MB in 8.66 seconds = 7.39 MB/sec

/dev/sdc:
Timing buffered disk reads: 64 MB in 10.88 seconds = 5.88 MB/sec

SCSI subsystem driver Revision: 1.00
sym53c8xx: at PCI bus 0, device 12, function 0
sym53c8xx: setting PCI_COMMAND_PARITY...(fix-up)
sym53c8xx: 53c875 detected with Symbios NVRAM
sym53c875-0: rev 0x26 on pci bus 0 device 12 function 0 irq 10
sym53c875-0: Symbios format NVRAM, ID 7, Fast-20, Parity Checking
sym53c875-0: on-chip RAM at 0xe0502000
sym53c875-0: restart (scsi reset).
sym53c875-0: Downloading SCSI SCRIPTS.
scsi0 : sym53c8xx-1.7.3c-20010512
sym53c875-0-<0,*>: FAST-10 WIDE SCSI 20.0 MB/s (100.0 ns, offset 15)
Vendor: IBM Model: DCHS09W Rev: 6363
Type: Direct-Access ANSI SCSI revision: 02
sym53c875-0-<1,*>: FAST-20 WIDE SCSI 40.0 MB/s (50.0 ns, offset 15)
Vendor: IBM Model: DGHS09U Rev: 03E0
Type: Direct-Access ANSI SCSI revision: 03
sym53c875-0-<4,*>: FAST-10 WIDE SCSI 20.0 MB/s (100.0 ns, offset 15)
Vendor: IBM Model: DCHS09W Rev: 6363
Type: Direct-Access ANSI SCSI revision: 02
sym53c875-0-<0,0>: tagged command queue depth set to 8
sym53c875-0-<1,0>: tagged command queue depth set to 8
sym53c875-0-<4,0>: tagged command queue depth set to 8
Attached scsi disk sda at scsi0, channel 0, id 0, lun 0
Attached scsi disk sdb at scsi0, channel 0, id 1, lun 0
Attached scsi disk sdc at scsi0, channel 0, id 4, lun 0
SCSI device sda: 17774160 512-byte hdwr sectors (9100 MB)
Partition check:
sda: sda1 < sda5 sda6 sda7 >
SCSI device sdb: 17774160 512-byte hdwr sectors (9100 MB)
sdb: sdb1 sdb2
SCSI device sdc: 17774160 512-byte hdwr sectors (9100 MB)
sdc: sdc1
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