Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!gandp!rg
From: rg@gandp (Dick Gill)
Newsgroups: comp.sys.ncr
Subject: The 8mm Gigabyte Gap
Message-ID: <343@gandp>
Date: 22 Feb 91 17:05:24 GMT
Reply-To: rg@gandp.UUCP (Dick Gill)
Organization: Gill & Piette, Inc.
Lines: 57


A client with a 32/650 (2.1.1) with about 880MB on-line disc
capacity (about 747MB used) recently purchased an NCR 8mm tape
drive (6091-2001-7190) with the thought that they could easily
back up their entire system to a single tape. That's what I
thought when I sold it to them, but I now have real doubts.

First some facts. The current status of the discs (from df -t)
is:

/usr/acct1      (/dev/dsk/15s1):    50972 blocks   64937 i-nodes
                   total:   572320 blocks   65488 i-nodes
/usr/acct       (/dev/dsk/14s1):    36688 blocks   60763 i-nodes
                   total:   572320 blocks   65488 i-nodes
/               (/dev/dsk/38s1):   179462 blocks   54000 i-nodes
                   total:   616874 blocks   65488 i-nodes

The tapes being used are 376 foot 8mm tapes from Exabyte (NCR
supplies does not have these tapes!!) Exabyte says that this is
the longest tape made for this drive.

The backup is run from crontab and uses cpio in a script that
has worked for me for years. (I did have to remove blocking
through dd to get it to run with the drive)

Now the story.  When the drive was first installed, their disc
usage was slightly higher than now, and they got error 28's (I
think I remember this correctly) near the end of the backup.
CODAR mulled it over for a couple days and finally decided
that the problem was that the "sparse" (aka "holely") files on
the system were being expanded by cpio and thus exceeding the
capacity of the tape.  While some large files of that type are
on the system, I had not experienced this problem before with
backups to regular 9-track or casette tapes.

Still, something was fishy; I didn't see how these files would
drive the tape past its (advertised) capacity of 2.1GB.  To
quote the NCR product writeup, "..this tape drive uses compact
cartridges to store up to 2.1 Gigabytes of user data on one tape
(maximum formatted capacity)."

To get a better handle on the problem, I said:

                  dd if=/dev/rmt/1yy of=/dev/null

and, when I got the results 6 hours later, guess what?  EOF
occurs at about 1.5 GB; got the same result with another new
tape! Is this right? Where is the missing 0.6 GB of capacity?

Inquiring minds (including my client) want to know.


-- 
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dick Gill     Gill & Piette, Inc.             (703)761-1163  ..uunet!gandp!rg

  


Brought to you by Super Global Mega Corp .com