Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!agate!bionet!apple!vsi1!daver!dlr
From: dlr@daver.UUCP (Dave Rand)
Newsgroups: comp.sys.nsc.32k
Subject: ICM-3216 disk speed
Keywords: System V ICM-3216
Message-ID: <7090@daver.UUCP>
Date: 8 Nov 88 02:00:57 GMT
Organization: Assn. for the prevention of Polar Bears and Kangaroos
Lines: 33

I have been trying to track down why the System V file system is
so slow, at least on the ICM-3216. I finally found the reason why.

I was getting about 40k bytes per second from the disk, through
the file system. This was confusing, as when a direct dd was
done, with a large block size, over 200K/second was possible.

I investigated a number of possibilities, and checked a number of
things. It came down to the fact that the Z-80 (which handles
the disk I/O on the ICM) had a latency of about 10 milliseconds
from the time it received a request, to the time it issued an
I/O to the SCSI device. This meant that the maximum I/O rate,
assuming zero seek time, zero rotational latency, and infinite
transfer rate was 100k/second (since System V requests 1k transfers).

To "fix" this, I modified the disk driver to cache requests,
and built a local 100k cache in the driver. This does a pre-read
if the block requested is not in the cache.

The transfer rate has improved dramatically, but still is not
as high as I would like to see it.

Does anyone else running the ICM-3216 have any ideas? 

Diffs for the changes are available, if you want to try them on
your system.

Disclaimer: These opinions reflect my thoughts only.



-- 
Dave Rand
{pyramid|hoptoad|sun|vsi1}!daver!dlr