using SCSI tape in read/write mode

[Bruce Germolus]

the first track.  Refer to the Viper(r) SCSI Models 60S, 125S, and 150S
Subject: Re: using SCSI tape in read/write mode
Summary: Don't update blocks on streaming tapes...
Expires: 
References: <755 at fnx.UUCP>
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Reply-To: bruce at triada.UUCP (Bruce Germolus)
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Organization: Triad Systems Corporation, Livermore, CA
Keywords: I'm confused

In article <755 at fnx.UUCP> del at fnx.UUCP (Dag Erik Lindberg) writes:
> [some background deleted]
>1) If I make a loop to write 30 x 1k records containing "Record #x",
>   rewind the tape, and read 30 records, printf()ing each, there 
>   is no problem.  Rewind again, read 15 records, write one record
>   containing "Record 15 modified" and rewind the tape.  Now read
>   30 records and printf().  Hmmm, nobody got modified.  The write()
>   did not return an error.  What am I doing wrong?
>
Updating blocks on streaming tapes of this type is *not* a good thing.
The update is not guaranteed to work due to the device's handling of
under-runs and bad blocks on the media (it re-writes duplicate blocks).
In addition, there is an erase head that is activated when writing to
the first track.  Refer to the Viper(r) SCSI Models 60S, 125S, and 150S
1/4-inch Streaming Cartridge Tape Drive "Theory of Operation and
Maintenance Manual" (Archive Manual Part Number 21981-001, rev B.,
sections 6.2.2, 6.3.2, and 6.8.1 contain the relavant material).
The only safe write operations to these devices is appending files to
the end of the tape or overwriting a tape with a new set of files.

>The system is ISC 2.0.2, the tape drive is an Archive 2150s on 1542.
>
With ISC 2.2, with respect to SCSI streaming tapes, write(2) will not
report errors it encounters (neither will read(2) for that matter).
These are known problems (at least here and at ISC).  My hunch is that
ISC 2.0.2 *MAY* contain the same problems, BUT I AM NOT SURE.
>
>-- 
>del AKA Erik Lindberg                             uunet!pilchuck!fnx!del
>                          Who is John Galt?
***************************Views*are*mine*not*TRIAD's***************************
Bruce Germolus                   Livermore, CA (USA) 94550-9559  
TRIAD Systems Corporation        415/449-0606                     
mavrik!bruce at lll-winken.llnl.gov 
-- 
***************************Views*are*mine*not*TRIAD's***************************
Bruce Germolus                   Livermore, CA (USA) 94550-9559  
TRIAD Systems Corporation        415/449-0606                     
mavrik!bruce at lll-winken.llnl.gov