Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!newstop!sun!aeras!tneale
From: tneale@aeras.UUCP (Tom Neale)
Newsgroups: comp.periphs
Subject: Re: SCSI->SCSI vs. SCSI->ESDI disk performance?
Keywords: SCSI ESDI MD21 Sun-3 disk performance
Message-ID: <359@aeras.UUCP>
Date: 19 Jul 89 20:06:55 GMT
References: <398@sunny.ucdavis.edu>
Reply-To: tneale@aeras.UUCP (Tom Neale)
Organization: Arix Corporation, San Jose, CA.
Lines: 41

In article <398@sunny.ucdavis.edu> poage@sunny.ucdavis.edu (Tom Poage) writes:

>Is there a performance hit in having an ESDI disk
>hooked to a SCSI bus through a translator (Emulex MD21)
>versus a SCSI disk hanging directly on the bus?

>ESDI disk performance is supposed to be higher than
>that of SCSI disks.  Does this mean that the two
>configurations are roughly equivalent in performance
>after taking into account the MD21?


Most manufacturers use the same HDA and servo systems on their SCSI
and ESDI products of a given form factor and capacity.  What this 
means is the controller is what causes most of the differences in
performance.

ESDI is a fairly low level interface while SCSI is a higher level,
"intelligent" interface.  SCSI drives often have lower performance 
numbers because of the command processing overhead required.  This 
is the time it takes the controller to decode, interpret and execute
a command.  This time is anywhere from 0.5 to 2.5 milliseconds.  Other
things that will affect performance of a SCSI drive are transfer rate
of the drive *and* controller (sync vs. async), disconnect capability,
read ahead caching and so on.  The combination of controller and drive
will affect performance as well because of the compatibility of SCSI
implementations.

On a single drive system you probably will see better performance with
and ESDI controller and drive.  If you have only one controller and 
multiple drives SCSI stands to give you better overall performance
because you can overlap operations.

How well will your ESDI drive work?  It depends on all the things
I've mentioned and probably on a few I've forgotten.  How's that for 
a long winded non-answer?
-- 
Blue skies,	| ...sun!aeras!tneale	| 
		| in flight:     N2103Q	|         The hurrieder I go
Tom Neale	| in freefall:   D8049	|         the behinder I get.
		| via the ether: WA1YUB	|