Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.arch
Subject: Re: Disk drives -- speed of?
Message-ID: <11116@mimsy.UUCP>
Date: 19 Apr 88 14:42:28 GMT
References: <2746@sundc.UUCP> <76700012@uiucdcsp>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 28

In article <76700012@uiucdcsp> gillies@uiucdcsp.cs.uiuc.edu writes:
>... A 3600 RPM disk rotates 60 times per second.  Thus, the maximum
>access time to any SECTOR on such a disk is 16.7ms.  The average
>SECTOR access time (assuming you start from a random sector) is 8.3ms.

[keep that assumption in mind!]

>Some of the more sophisticated stepper motors (voice-coil?) in
>Winchester disks can do an average TRACK seek in 16.5 ms (CDC Wren).
>But why make the disk spin faster, if the head cannot get to the TRACK
>quickly?  The average TRACK seek time would have to be 8.3ms before it
>would be the main bottleneck in disk random-access performance.

The `average access time' is not necessarily the average access time.
Each of these averages assumes you are starting from a random place.
The whole point of the 4.2BSD `fast file system' (and any other
optimised allocation scheme) is to eliminate that assumption.  The
4.2BSD block allocator tends (somewhat) to allocate out of a single
cylinder, allocating rotationlly-optimal sectors.  (`Rotationally-
optimal' to an approximation, at least.)  It also tends (more than
somewhat) to allocate out of a single cylinder group (reducing average
track seek time).

Large contiguous block allocators (`extent systems') have the same
sort of effect (possibly moreso).
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris