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From: kev@hpcpbla.HP.COM (Kevin Jones)
Newsgroups: comp.periphs.scsi
Subject: Re: DAT versus 8mm Tape
Message-ID: <9850006@hpcpbla.HP.COM>
Date: 9 Nov 90 07:54:15 GMT
References: <272DB1E1.9737@orion.oac.uci.edu>
Organization: HP Computer Peripherals Bristol, UK
Lines: 91

> The EXB-8500 has a faster seek time than DAT.

My understanding is that the EXB-8500 achieves a fast "seek"
by the following process:

1. Host writes data to the drive
2. During the write operation (or a subsequent read pass at
   normal speed), the host may as the drive to report current
   physical tape position (as a count of servo pulses).
3. When a "seek" to a specific record is required, the host
   can command the drive to "position to XX servo pulses from
   start of tape".

This is not a seek function. It is a goto function.
I have no doubts that it can prove just as effective as DAT seeks,
PROVIDING the servo pulse count information (call it INDEXING
information) is readily available whenever a fast "seek" is
required.

In order to utilise this ability you will need

* A driver cabable of "gathering" indexing information as it writes.
* A driver cabable of preserving ?somewhere? this information until
  it is needed.
* A driver capable of reading and associating this information with
  some requested logical position, then issuing the appropriate command
  to the drive to "get there".

- ie. A highly Exabyte specific custom driver.

I would like to point out that DDS-DAT implements a GENUINE fast
search. DDS drives maintain counts of "records so far",
"filemarks so far" and "setmarks so far" which they embedd in the
data as it is written to tape. These counts can be read by the
drive as it seeks at up to 200X normal speed. Any host driver capable
of issuing a SCSI 'SPACE' command (STANDARD SCSI) can therefore
reap the benefits of DDS fast search.


DATA COMPRESSION:

> but then you get into the pros and
> cons of compression and data reliability

There are no pro's and cons of data integrity / error rate / reliability
with the data compression being used on HP's (and other) DDS drives.
HP drives (DC and non DC) both have error rates of
LESS THAN 1 BIT IN EVERY 10^15 BITS UNCORRECTABLE.

Data integrity is not compromised by the addition of DC.
The adaptive DC algorithms being used mean that the argument
"trash your dictionary and you trash your tape" no longer holds.
Adaptive DC algorithms reset their dictionaries repetitively.

The argument "with 4X compression, loosing 1 bit on tape means you
loose 4 bits at the host" does apply, but bear in mind that with
data compression, LESS DATA ON TAPE is used to store the SAME AMOUNT
OF HOST DATA. A given quantity "N" of host data is therefore
"X" times LESS likely to suffer from an uncorrectable error.
If an uncorrectable error does occur however, then instead of
1 bit in error, you've got "X" bits in error. The net result is
that the error rate is unchanged.

(This analysis is a bit simplified, the nature of ECC schemes used
 in drives means you don't go loosing individual BITS. Instead, you
 typically fail to correct a BLOCK.

 The truth is, that with DDS's 1 in 10^15 error rate, you are more
 likely to loose data thru misuse of tape/drives than due to the
 "natural" error rate of the drive. eg. store tapes in high humidity,
 store tapes on top of your non-magnetically screened display,
 use tapes beyond their specified life, fail to adhere to
 reccomended maintenance practices such as head cleaning ......)


Another attempt at an unbiased opinion from a DDS develpment
engineer...............

-----------------------------------------------------------------
Kevin Jones.   (in DATland)        | Hewlett Packard Ltd,
                                   | Computer Peripherals Bristol,
kev%hpcpbla@hplb.hpl.hp.com        | Filton Road,
                                   | Stoke Gifford,
Tel: 011 44 272 799910 (ext 22351) | Bristol.   BS12 6QZ.
                                   | ENGLAND.
-----------------------------------------------------------------

This response does not represent the official position of, or
statement by, the Hewlett-Packard Company.  The above data is
provided for informational purposes only. It is supplied
without warranty of any kind.