From: guyd@austin.ibm.com (Guy Dawson)
Subject: Re: Cached Hardrives
Originator: guyd@pal500.austin.ibm.com
Organization: IBM Austin
Lines: 37


In article <1993Apr23.142720.25002@spartan.ac.BrockU.CA>, tmc@spartan.ac.BrockU.CA (Tim Ciceran) writes:
> 
> After reading many postings concerning hard drives I'm still unclear
> about one thing.  These new "cached hard drives" - is the cache on the
> drive itself or is it separate, like on the controller or something?
> Thanks to anyone who can clear this up.

I have two SCSI hard drives, each has 512KB of 'cache'.

There several reasons for this :-

When the drive has read requested data from the disk the SCSI bus may be
busy. This data needs to be stored some where until the bus is free and
the data can be transmitted.

When the drive receives data to be written, the data can come down the bus
faster than the drive can write it to the disk. It needs to be stored
somewhere while the disk is writing it out.

In these situations the memory is being used as a buffer NOT a cache!

May be the drive does some read-adead caching as well...

> 
> -- 
> 
> TMC
> (tmc@spartan.ac.BrockU.ca)
> 

Guy
-- 
-- -----------------------------------------------------------------------------
Guy Dawson - Hoskyns Group Plc.
        guyd@hoskyns.co.uk  Tel Hoskyns UK     -  71 251 2128
        guyd@austin.ibm.com Tel IBM Austin USA - 512 838 3377
