From: gezelter@rlgsc.com
Newsgroups: comp.sys.dec,comp.os.vms
Subject: Re: DUPLICATING TA90s ???
Date: 12 Aug 92 06:31:29 EST
Organization: Robert Gezelter Software Consultant, Flushing, NY

In article <1992Aug11.123646.20532@ultb.isc.rit.edu>, ecldco@ultb.isc.rit.edu (E.C. Loyd) writes:
> [lots of stuff relating to copying TA90s deleted]
> 
>>Gentlemen,
>> 
>>A warning! It is important to note that when you copy tapes, 
>>error blocks may appear/disappear. If you are copying VMS BACKUP 
>>savesets, it is extremely important that you leave on the CRC 
>>checking and redundancy grouping capabilities of VMS BACKUP. 
>>Otherwise, there is a distinct chance that your saveset may get 
>>corrupted. 
>> 
>>While it is often painful, precautions are necessary to ensure 
>>that the savesets you copy (and take off of the site) are usable 
>>in the event of a problem!
>> 
>>- Bob
> 
> But what we here at RIT would like to do it copy, bit-for-bit, one tape to
> another tape.  There would be no loss of data in this fashion.
> 
> -Eric
> 
> -- 
> ECLDCO@RITVAX.bitnet       ECL6895@RITVAX.bitnet
> ECLDCO@ultb.isc.rit.edu    ECL6895@ultb.isc.rit.edu
> Eric Loyd's the name, Data Center Operations is the game.
> Opinions and irrational outbursts are MY pride and joy.  Not my employer's.
-- 
Eric,
 
My apologies. My original post was not specific enough.
 
The problem that I am referring to will happen on may hardware 
combinations. While it is possible that this scenario will not 
happen on a TA90, it does happen on other drives (and, in fact, 
it may happen on TA90s anyway, this particular case is one of 
those "its not supposed to happen" situations).
 
The problem occurs when a tape drive senses an error when writing 
the tape and processes a "recoverable" tape error. When this tape 
is subsequently copied using a different drive (or the same drive 
at a different time) the condition which caused the first error 
may not be detected by the tape copy program, resulting in two 
copies of the tape record involved. If the first record was truly 
in error, and the copy program (or the tape drive) did not handle 
it correctly, then the "duplicate" of the tape will have an 
erroneous block (On the original tape, the hardware error was 
used to mark the problem, since the duplicate tape has no 
hardware error, there is NO PROBLEM DETECTED).
 
In some environments, this is not a particular problem (Tape 
distributions of software use brand new media, the copy operation 
is large scale, and a replacement for a bad copy can always be 
obtained). For archival backups, where the primary and backup 
copy ARE THE ONLY copies of the data, I would recommend more 
caution.
 
I hope that this has been clearer. 
 
- Bob
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| Robert "Bob" Gezelter                       E-Mail:  gezelter@rlgsc.com  |
| Robert Gezelter Software Consultant         Voice:   +1 718 463 1079     |
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Newsgroups: comp.sys.dec