Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!mimsy!umd5!ames!ucbcad!ucbvax!cbosgd!mandrill!nitrex!rbl
From: rbl@nitrex.UUCP ( Dr. Robin Lake )
Newsgroups: comp.unix.wizards
Subject: Re: /dev/swap - possibility of it being a ramdisk
Message-ID: <586@nitrex.UUCP>
Date: 16 Dec 87 13:21:45 GMT
References: <712@qetzal.UUCP> <585@nitrex.UUCP> <17013@topaz.rutgers.edu>
Reply-To: rbl@nitrex.UUCP ( Dr. Robin Lake )
Organization: The Standard Oil Co., Cleveland
Lines: 28

In article <17013@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes:
>Say what?  On PDP-11's UNIBUS attached disks can not possilby have
>17,000 times the transfer rate of conventional disks.  The whole
>bandwidth of the bus is 2.2MB per second in the best of cases.  Most
>drives even in days gone by could approach a 1MB/s.  What you don't
>have is the seek latency.
>
>-Ron


It depends upon what the transfer rate and interleave factor of the 
"conventional disks" are.  The solid-state RF-11/RS-11 disk equivalent
was invented in the summer of 1970.  There were few head-per-track
disks from DEC at that time (as I recall, the one adapted from the PDP-8
and PDP-12 lines was 32Kbyte or 64K bytes) and the low bit density led
to slow transfer rates.  So.... it's all a question of the denominator.

The controller on the Monolithic Systems "EMU" (Extended Memory Unit) was
capable of doing multiple DMA disk transfers in a single bus request, which
pushed the transfer rate right close to the Unibus limit.  I've forgotten
what we got for the peak transfer rate, but I vaguely remember a hair above
2 Mb/sec.  (8 times 256 K words in a data acquisition application using
a dual-port "EMU").

Rob Lake
-- 
Rob Lake
{decvax,ihnp4!cbosgd}!mandrill!nitrex!rbl