[Info-vax] compressed SDLT capacity

[B Hobbs]

I'm wondering how much data I can stuff onto a SDLT cartridge on my
system.  The following is my SWAG of an answer.

System configuration:
  OpenVMS V7.1
  VAX emulator Charon-VAX/XL (Plus) V 3.1 B 50 (VAX 4000 model 108)
    $ show cpu lists VAX 4000-105A
  Windows Server 2003 SP1, v 5.2
  HP ProLiant DL385, AMD 64 Opertron
  HP 160/320 GB SDLT, SCSI connection
    actually a Quantum SDLT 320 under the bezel
    VMS thinks the SDLT is a TK50 (about seven generations apart?)

It takes my system almost three hours to complete a INITIALIZE/ERASE
of an SDLT I cartridge in the HP SDLT 320 drive.  The backup script
writes some 30 files totaling 40 GB to the SDLT each night.  A
DIRECTORY of the SDLT with one night's worth of backup takes about
eight minutes, two night's about sixteen minutes, etc..  I have not
yet filled up an SDLT cartridge.

I assume that when I do a INITIALIZE/ERASE, VMS sends a SCSI erase
command to the drive and VMS does other stuff until the drive signals
that the erase command is complete -- the drive does the work and the
SCSI bus is quiet.  If this is correct, then the equivalent of VMS's
INITIALIZE/ERASE on other OSes would also take almost three hours for
a SDLT I cartridge in a HP SDLT 320 drive.

I am using an emulator.  Can we get a few figures from non-emulated
VMS systems and other OSes?  Maybe list the drive make and model,
cartridge type, OS, hardware, and the amount of time to INITIALIZE/
ERASE (or equivalent command).

How does a DIRECTORY of the SDLT work?  Does the tape drive send all
the data to VMS and VMS sorts thru the data to assemble something like
a directory listing?  That would be 40 GB per backup set going over
the SCSI bus.  Or does the drive look for specific directory entries
on the tape and just send those?  That would be about 30 directory
entries per backup set for my system, probably a couple megabytes.  Or
is there something else going on?  Somewhere I saw that there is a
directory area at the beginning of the tape, but I suspect that the
main data area of the tape is involved because the more data I put on
the tape, the longer a DIRECTORY takes.

Let's see if I can figure out how much of my data a SDLT cartridge can
hold ... with a few assumptions.

The Quantum SDLT 320 specs ( http://downloads.quantum.com/sdlt320/8185002A04.pdf
) are

  tape length is 1800 feet, 1765 feet usable for data

  448 physical tracks, 56 logical tracks (8 track head)

  read / write tape speed of 122 inches per second

1,765 feet x 12 inches per foot = 21,180 inches of tape

21,180 inches x 56 logical tracks = 1,186,080 inches of logical track
per tape

1,186,080 inches / 122 inches per second = 9,721.97 seconds to go from
beginning-of-tape (BOT) to end-of-tape (EOT)

9,721.97 seconds / 60 seconds per minute = 162.03 minutes BOT to EOT

162.03 minutes / 60 minutes per hour = 2.70 hours BOT to EOT

I suspect it takes a few seconds to reverse direction at the end of
each of the 56 logical tracks, so three hours (180 minutes) should be
pretty close.  It does say that read and write are the same speed, and
I assume that the INITIALIZE/ERASE and DIRECTORY commands are working
at full speed with no stops.

162.03 minutes / 8 minutes to DIRECTORY one backup set = 20.25 backup
file sets per tape on my system

Wow!  I think the max number of backup sets on a tape has been five or
six before the tape was INITed.

20.25 backup file sets * 40 GB in my backup file set = 810 GB per tape
on my system

810 GB / 160 GB uncompress SDLT capacity = 5.0625 : 1 compression on
my system

If I did this correctly, I should be able to put about 800 GB of my
backups on one SDLT I cartridge.  My backup script does a DIRECTORY
before and after the backups, so I should be able to track how full
the tape is by the amount of time the DIRECTORY takes.

I found the specs in Standard ECMA-320 (
http://www.ecma-international.org/publications/standards/Ecma-320.htm
) to be close to Quantum's.

I await someone to point out where I screwed up my calculations or
assumptions.

I plan to verify that I can get around 800 GB on a SDLT.  Let's
see ... 20 backup sets ... 13 cartridges on hand ... change cartridge
5 times a week ... errr, carry the square root of -2 ... it'll take
about a year to fill up those cartridges.

-- Bill Hobbs