Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!ames!killer!texbell!uhnix1!moray!siswat!buck
From: buck@siswat.UUCP (A. Lester Buck)
Newsgroups: comp.periphs
Subject: Re: Impulse drives from Quantum/Plus
Summary: transfer overhead != transfer time
Message-ID: <381@siswat.UUCP>
Date: 17 Mar 89 04:55:49 GMT
References: <1397@ccnysci.UUCP> <379@siswat.UUCP> <7657@polya.Stanford.EDU>
Organization: Photon Graphics, Houston
Lines: 33

In article <7657@polya.Stanford.EDU>, kaufman@polya.Stanford.EDU (Marc T. Kaufman) writes:
. In article <379@siswat.UUCP> buck@siswat.UUCP (A. Lester Buck) writes:
. >I didn't read the Infoworld article, but does this guy work for 
. >the company making Cluster Disk Interfaces?  He sure doesn't
. >understand SCSI...
. 
. >            ...It turns out that the Unix buffer cache search is
. >about 30% of the total transfer overhead today, and the
. >requirement that the target check every bit of the SCSI
. >command data block parameters for validity is another 30%
. >or so...
. 
. Huh?  The command data block is 6 bytes long.  The only parameter of any
. consequence in it is the sector address.  How can this take 30% of the transfer
. time?  The Mac II SCSI interface peaks at 1.4 MB/sec transfer rate.  The SE can
. get 600 KB/sec.  Using the Device interface on a IIx, I get 330 KB/sec
. sustained on Apple's HD 80, and 450 KB/sec on a new SuperMac drive.  As this is
. a continuous read through 80 MB of disk, the cache is of no consequence in
. these numbers.

The article I referenced measured SCSI *transfer overhead*, i.e., the
wasted time in a transfer over and above the basic transfer rate of
1.5 MB/sec for asynchronous or 4-5 MB/sec synchronous.  Checking every
reserved bit in the 6 bytes can chew up a few hundred microseconds
on a slow target controller.  The total overhead for a single transfer
is on the order of three milliseconds total today, with lots of room
for improvement with improved implementations.  You can figure out
what your overhead timing is per SCSI read/write op on your Mac -
just figure the difference between 1.4 MB/sec and your observed throughput
of 600 KB/sec.  (You need to know the blocksize, too.)

-- 
A. Lester Buck		...!texbell!moray!siswat!buck