Re: /disk0, was MDC1-GC:lessons learned

[Arie Shoshani <arie@math.lbl.gov>]

> If the 2 MB/s transfer rate is to /disk0 on rmds03, I believe I have
> tracked down the source of the problem.
> 
> At some point, the SCSI bus that connects the drives in the /disk0
> array renegotiated the transfer rates between the SCSI controller on
> rmds03 and the individual drives. Some drives are at 10MB/sec, others
> are at 20MB/sec, some are running SCSI Narrow, others are running 
SCSI 
> Wide. None are running at their rated 40MB/sec.
> 
> As a result of this asymmetry, Online Disk Suite and the SCSI device
> driver was left with the complex task of reconstructing the stripe
> writes and reads. This explains the high utilization and low
> read/write performance reported by the ODS monitoring tools and
> zoom.se.
> 
> Unfortunately, I didn't think of this earlier and only just today
> installed the tool that verified the problem.
> 
> At this point, I know of no tools that will reset the SCSI bus and
> force renegotiation between the drives and the SCSI controller. As a
> result, the problem will only be fixed when the system is
> rebooted. (Note that the problem only affects /disk0)
> 
> 
> Sorry about that.
> 
> Shigeki

Glad to hear you tracked this down.  It seems useful to have some tools 
that monitor network traffic dynamically.  As far as our test are 
concerned, they are still valid since the slow transfer rate was pretty 
consistent, and thus the comparison is scaled down across all tests.

Thanks, Arie.