[comp.os.vms] Helical Scan Tape unit evaluation

TNIELAND@FALCON.BERKELEY.EDU (Ted Nieland - SRL) (05/07/88)

For those that are interested, I have just installed one of the 8mm helical
scan tape drives.  This one, from Summus Computer Systems, uses the EXABYTE
tape unit. 

IMPORTANT NOTE:  I am interested in "trading" tapes with anyone else who has a 
8mm helical scan tape drive.  I would like to check out the transportability 
of the tapes between manufacturers.  Anyone who has a drive from any of the 
other vendors and who would be willing to trade a tape with me (with some dummy 
data in backup and copy format) please send me a message.  If you are going to 
DECUS in Cincinatti, we could trade them their.  I will summarize my findings 
to INFO-VAX and in an article in the PAGESWAPPER.


The following is the information on the unit, my preliminary test results, and 
some personal observations:

For those who do not know what a helical scan drive is:  A helical scan tape 
drive is uses Video tape or DAT (Digital Audio Tape) and writes the data in 
stripes that cross the tape at an angle.  The read/write head is on a rotor 
that spins, therefore the tape to read/write head speed is much higher than 
the tape speed.

Most of the 8mm drives I have priced are around $7K or lower.  The 8mm video 
tapes can be bought from approx $9.00 a piece commercially.

Information from Summus.

Tape Unit:
	Transfer rate (peak): 1.5 mb/sec
	Transfer rate (sustained):  246 kbytes/sec

	Buffer size (tape): 256 kb

	Tape motion speed:  0.429 ips
	Rotor speed 1800 rpm
	Effective head to tape speed 150 ips max.
	Rewind speed: 75 times nominal
	File search speed 10 times nominal.
	Nonrecoverable error rate: less than 1 in  10**13 bits

A 2048 size 8mm cartridge (120 minute video) can store 2.332 Gb.

Unibus Controller:

	DMA Data rate up to 2.0 mb/sec peak
	SCSI data rate in excess of 1.25 MB/sec asynch


The tape drive looks like a TK-50 to the VAX.

The unit is hooked up to a VAX 8650, 32 MB memory with 7 RA81s attached.  

The system disk is attached to a UDA50 on the same UNIBUS as the tape unit.

I have done several transfers using backup.  The command was of the form:

	$ BACKUP/IMAGE/BUFFERS=5/NOCRC/BLOCKSIZE=16384 disk: MUA0:disk.BCK

The following are the results:

 Disk   Number of blocks     Approx time     Rough Kbytes/sec

  A	14811 (243 Mb)		32 min		126
  B	16040 (263 Mb)		32 min		136
  C	16670 (273 Mb)		40 min		113
  D	22415 (367 Mb)		41 min		149
  E     15920 (260 Mb)		29 min		150

These test were done with a light load on the system (20-30 users).
Disk A is the system disk.  The other disks were on a separate UNIBUS (DW2)
than the tape unit.

We are currently doing BACKUPs on a TU81 Plus at 6250 BPI and each of the 
disks above take approx 1 hour to BACKUP.

The one tape was holding the 1.4 Gb, but I couldn't tell how full it was.

Comments:

1)  Initializing the tape is just like a TK-50.  It sits there for a
    minute or two before doing anything (I think this is the DEC driver's
    fault).

2)  Mounting the tape is just like a TK-50.  It sits there for a
    minute or two before doing anything (Again, I think this is the DEC 
    driver's fault).

3)  Dismounting takes quite a while since it has to rewind the tape first.

4)  When the unit does an unload on DISMOUNT, the tape cartridge is ejected.
    The door actually pops open and throws the tape part way out.

5)  Installation was fairly easy and painless.  I experienced no problems
    (so far).

6)  The unit is small (measures approx 7" by 15" by 5.5" in a "shoebox"
    configuration) and is lightweight.  It can set on top of the CPU cabinet.

7)  Causes new problems:

    a)  Easier for people to walk off with tapes without you noticing.
        Security needs to be tighter on tapes.

    b)  How do you proper label such a small tape that contains so much
        data?

    c)  What do you do with the operators that you have changing the
        tapes now?

8)  Searching the tape for a particular saveset does take a few minutes, but
    considering the amount of data, it is fast.

	
Disclaimer:  I am not employed by Summus Computer Systems, nor do I have any 
vested interest in the company.  The previous is an evaluation of a product 
recently purchased that I thought people may be interested in.


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