Newsgroups: bit.listserv.ibm-main
Date:         Tue, 8 Sep 1992 01:00:00 PDT
From:         Leonard D Woren <LDW@USCMVSA.BITNET>
Subject:      Re: PROC TAPELABEL and 3490E drives

> The original 3480's were:
>  1.) 18 track
>  2.) 550 feet long

We've been over this before on this list...

The manual states that the usable area of a 3480 cartridge must be at
least 150 meters.  That's 492 feet.  The fact that some vendors are
selling longer cartridges is an aside.  A "normal" length cartridge is
typically 3-5% longer than the 492 feet, making it 510 or so feet.
All of the cartridges in *our* library that I checked when I was
adding the IDRC code to TAPEMAP showed 103-105% full, with TAPEMAP
using 492 feet as 100%.  So I have faith in my (and TAPEMAP's)
arithmetic.

>  3.) 38k bpi (density)

Actually, according to the manual, it's 1491 bytes/mm, which is 37871
bytes/inch.

>  4.) .08 inch inter-block gap

I don't think the IBG is given in the manual, but everyone seems to
agree that it's .08 inch.  What I'd still like to know is how much
tape a tapemark takes.  The reason why I'm concerned about this is
that on a 9 track, a tapemark is followed by a 3.75 inch erase gap.

> and a CART could hold about 240 mb at max.

The manual states 200 mb.  This is no doubt figured using the minimum
required length, and a lower than maximum blksize.

> I'm told that a later incarnation of the 3480 implemented data compression
> with hardware. Was it different in any other way?

I don't know if IDRC (Improved Data Recording (what does the C stand
for?)) is a new version so much as a later add-on feature for 3480s.

> What were the major differences of the original 3490 drives?

IBM fixed the design problems in the 3480?  :-)
IDRC is standard, I think.

> Did the Blue gentleman say that the 3490E CART is 1100 feet long and that
> the drive reads/writes 18 tracks in each tape direction?  With double the

Well, the 3490E _does_ write 18 tracks in each direction.  This means
that if you're filling a series of cartridges, the rewind time is
zero!  I had heard somewhere that there would be double length
cartridges available.  Can someone with 3490Es confirm this?

> physical length and double the number of tracks (total) of the old 3480,
> does this mean that a 3490E CART can hold around 960 mb BEFORE compression?

Probably more like 800mb or so, depending on what length you figure on
and the blksize.  Note that a blksize of 3040 on a cartridge wastes
HALF of the tape in IBGs.  IEHMOVE's hardcoded output blksize of 800
wastes 80% of the cartridge.  A blksize of 80 (unblocked card images)
wastes 99.8% of the cartridge.  32K blksize is normally appropriate
(and only wastes 10%...)  FDR (and maybe DFDSS; I don't remember) uses
larger block sizes, in order to fit a whole track image into one tape
block.

Now, I have a question.  Does IDRC use very large blocks (to reduce
the amount of tape wasted in IBGs?)

> If so, I, for one, am impressed.

So am I.  Now, if you figure an "average" compression ration of 3.5:1,
one of those suckers should just about hold a full dump of a 3390-3.
IBM was getting concerned about how many volumes were required to dump
their largest disks.  It looks like the 3490E addresses that problem.


For those familiar with GIF, an interesting data point.  GIFs are
supposedly compressed using a fairly good algorithm.  And I'd guess
that IDRC uses a fairly good algorithm.  I put a bunch of GIFs on a
cartridge for a friend of mine, and just for amusement I specified
compression.  My new TAPEMAP said "overall compression factor for
this tape is 1.0"!  I've seen other files with compression factors up
to 11, so this was somewhat amusing.

/Leonard
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