Newsgroups: comp.periphs.scsi

>     1. How long is a filemark (in terms of data capacity)? 

4 bytes.

>        Does the length of 
>        the filemark introduce any positioning restrictions?

No. There is no such thing as "short" and "long" filemarks. Just a filemark.
The only drawback is if you write it "Non Immediate" (as defined in SCSI)
which forces buffered data to tape. This can result in a waste of capacity
in some drives (max waste = 126Kbytes). The application/os-driver is 
responsible for immediate/non-immediate filemarks. HP drives can be configured
to always write "immediate" marks by setting switch 6 on the underside of the
drive to "0". This can result in performance gains on systems that write lots
of filemarks in non-immediate mode. (Note: no positioning restrictions 
are introduced).

>     2. How many times does a DAT retry a read before reporting a media error?

      Depends on the drive in question. I'm not too sure about the number of 
      attempts, but an HP drive will spend about 10 minutes before abandoning 
      a read.

>     3. Does a DAT, like an Exabyte, rewrite data when it detects an error in
>        data it has just written? 

      Yes.

>     Can it detect such errors in the first place?

      Yes.

>     4. Is there the same kind of ongoing discussion about the merits of data-
>        grade and the demerits of consumer-grade tape? And if so, are there any
>        real conclusions? :-)

      There has been an ongoing discussion. Lots of individuals are drawing 
      their own conclusions (some as a result of bad experiences).
      The "official" HP line is that you should only use "DDS" branded media.
      I would categorise DAT media as follows:
         1. DDS branded. Media has DDS logo with the words "Digital Data 
            Storage". A manufacturer must have his media approved by 
            HP/Sony before branding it DDS. Approval involves extensive 
            media testing.
         2. Media marked as "Data Grade" without the DDS logo. The words on 
            the sticky label are no guarantee of media quality. I would avoid
            such tapes as you don't know what you're getting for your money.
         3. "Known to be Good" branded media such as Maxell or Sony.
            If you want to save some $$$ and not get burnt too badly then
            this type of media is a good option. A lot of folks seem quite 
            happy taking this route. 
         4. "Never heared of that name before" media. This is likely to 
            be cheapest to purchase. If however, you place some value  on
            your data, it could turn out to be fairly expensive in the long
            run.
            
            
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Kevin Jones.                       | Hewlett Packard Ltd,
                                   | Computer Peripherals Bristol,
kev%hpcpbla@hplb.hpl.hp.com        | Filton Road,
                                   | Stoke Gifford,
Tel: 011 44 272 228351             | Bristol.   BS12 6QZ.
                                   | ENGLAND.
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Newsgroups: comp.periphs.scsi