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From: ghg@ecn.purdue.edu (George Goble)
Newsgroups: comp.unix.admin
Subject: Re: Dumping to an exabyte tape drive
Message-ID: <1990Sep12.151518.18944@ecn.purdue.edu>
Date: 12 Sep 90 15:15:18 GMT
References: <439@cfa.HARVARD.EDU> <776PKVH@dri.com> <1088@flash.UUCP>
Organization: Purdue University Engineering Computer Network
Lines: 27

In article <1088@flash.UUCP> klg@flash.UUCP (Kevin L. Gross) writes:
>
>Although other data, specifically that from Eakins Assoc., makers of
>the SPERAD (Scsi to PERtec Adapter board) interface, say that the
>filemarkers are the equivilent of 2.2 MB, however, the dump program may
>be using a much larger file mark than the filemark SPERAD considers normal.
>(So sayeth Sequent anyway)
>
>Hence, when you dump to 8mm tape the way I do, with about 17 filesystems,
>you only get about 1.8 GB per tape instead of the advertised 2.3 GB. I'm
>not exactly sure of the amount I lose per filemarker, but I do know if I
>try to put more than 1.8 GB on the tape, I get a hung tapedrive because
>it ran out of tape.
>

LONG EOF markers (default) are about 2.2MB.  Exabyte has an LEOT warning
before the PEOT (physical eot).. Older F/W has this about 235 or 240 MB
from the end of tape.  This coupled with "dribbling in" the data, so the
drive wastes blocks starting/stopping, can easily result in only 1.5GB.
The worst data rate is one that makes the drive "almost stop", it writes
the pad blocks, but more comes in, so it keeps going, writes more pads,
etc.  Faster or slower aggregate rates (streaming or drive stopping)
are much better.. worst start/stop loss is 100-200 MB on a whole tape.
Newer (4$25 or later MX F/W), has LEOT only 30MB or so from PEOT, not
240 MB.  A smart driver can write thru the LEOT.  Each write returns
a SCSI check (warning), but the write works.
--ghg