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SubjectRe: Poor SCSI drive performance on SMP machine, 2.2.16


On Sun, 28 Jan 2001, paradox3 wrote:

> I have an SMP machine (dual PII 400s) running 2.2.16 with one 10,000 RPM IBM
> 10 GB SCSI drive
> (AIC 7890 on motherboard, using aic7xxx.o), and four various IDE drives. The
> SCSI drive
> performs the worst. In tests of writing 100 MB and sync'ing, one of my IDE
> drives takes 31 seconds. The SCSI drive (while doing nothing else) took
> 2 minutes, 10 seconds. This is extremely noticable in file transfers that
> completely
> monopolize the SCSI drive, and are much slower than when involving the IDE
> drives.
> After a large data operation on the SCSI drive, the system will hang for
> several minutes.
> Anyone know what could be causing this? Thanks.
>
> Attached are some data to help.

You didn't provide enough information for anybody to have a single idea
about the cause of the problem you report, in my opinion.

Just any not too old 10,000 RPM disk is able to sustain more that 25 MB/s
sequential data transfer, but cannot do better than 5 milli-seconds
latency with random IO patterns. So, result for 100 MB transfer can be
less than 4 seconds in the best case but greater than (25000*5)/1000=125
seconds for random 4K IO pattern, for example.

What you want to do, in my opinion, could be:

- Check in the syslog the actual transfer speed that has been negotiated
for your SCSI disk.
- Also check if error messages related to disk IOs have been reported by
the kernel.
- Run some benchmark to check, at least, sequential IO performance (iozone
for example will fit)

Gérard.

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