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From: olson@anchor.esd.sgi.com (Dave Olson)
Newsgroups: comp.periphs,comp.sys.sgi,comp.periphs.scsi
Subject: Re: exabyte record size limit
Message-ID: <1991Feb22.202621.10622@odin.corp.sgi.com>
Date: 22 Feb 91 20:26:21 GMT
References: <1991Feb8.152435.10875@helios.physics.utoronto.ca> <1991Feb14.223941.4318@b11.ingr.com> <1991Feb22.165448.11408@fy.chalmers.se>
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In <1991Feb22.165448.11408@fy.chalmers.se> co@fy.chalmers.se (Christer Olsson) writes:
| We have a uVAX II/GPX with an IBIS 1.2 Gigabytes videodisc. The
| Imageprocessing system backups the disk into some hundreds of savesets
| (one saveset contains one picture and a desciptorfile.).
|  
| But, can an Exabyte handle so many savesets without loosing capacity?
| I think a complete backup generate more than 600-700 savesets.

You lose aproximately 2 Mb per filemark with the (more common?) long
FM's, and I assume your savesets are seperated by FM's.  This means
about 1/2 the tape would be used by FM's.  Some drivers/manufactures
default to short FM's, which use less tape, but which don't allow you
to append at arbitrary points.  It sounds like that would be fine for your use.
(The SGI tape driver writes only long FM's, by the way).

| Is it possible for a slow uVAX II vith VMS 4.7 backup to feed an
| Exabyte with enough data without loosing space? I think an Exabyte
| needs at least 100K/byte per second under writing a saveset? (no saveset is
| smaller than 400K, some savesets is 300 Mbyte large) 

Can't help you with the uVAX, but to keep the Exabyte streaming continuously,
you need to feed it data at about 240 Kbytes/sec.
--

	Dave Olson

Life would be so much easier if we could just look at the source code.


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