Newsgroups: comp.periphs.scsi

>> To this end, I've been experimenting with compression during backup,
>> and as I had already discovered compression and trying to stream the
>> tape drive are not compatible.

This depends a lot on the type of system you are running on.  You give
no indication of this in your posting so I have nothing to go on.  If
your CPU cannot compress data as fast as the tape drive can accept it,
then the tape drive will not stream.  This is a function of the
transfer rate of the drive and the speed of the CPU.  Other things can
affect it such as the overheads involved in disc accesses, the tape
driver and the tape drive itself.

>> With this new DAT drive, is it as important to try to keep the drive
>> streaming as it was with the 1/4" drive?

DAT drives will wear out faster if they do not stream.  The main
failures are due to actual tape pulling time which affects head wear,
repositions which affects wear in the tape pulling mechanism and
tapeloading which affects wear in the tape load mechanism.  If the
tape does not stream, then this rapidly becomes the dominant factor.
How significant it becomes, I do not know.

>> As an example, I did a backup of a directory without compression
>> that took 20 minutes to run, but kept the drive streaming (at least
>> I assume it was streaming, since the activity light was flashing
>> the whole time - the drive is so quiet, I can't *hear* it do
>> anything).

This is no indication that the drive is streaming.  It is only an
indication that the host is sending data to the drive.  It is
virtually impossible to tell if the drive is streaming.  Even with the
lid off you have to be very careful because you don't notice the
reversals in motion.

>> If I turned on compression, the backup took only 8 minutes total
>> elapsed time, presumably since a lot less data had to be written to
>> the tape (this particular directory average 70% compression), but the
>> drive did not stream (again, as indicated by the activity light).

Since the activity light stopped flashing, the drive was waiting for
the host to send it more data.  This would suggest that the limiting
factor in the system is the time taken for the host CPU to compress
the data.

>> I checked the manual that came with the drive, and it doesn't seem
>> to have anything in it pertaining to streaming and the life
>> expectancy of the drive.

There are life specifications, but I do not know whether they have
been published or not.  I would tend not to be too bothered about
repositions.  I do not think they have as big an effect as for QIC
drives.  I would suggest you consider an HP35480 DAT drive with built
in hardware compression.  This would take the load off the host and
improve the performance of the system considerably.  (It would also
help pay my salary, so take my comments with that in mind :-)

	Mark Simms

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Opinions expressed are my own and are not intended to be an official
statement by Hewlett-Packard Company
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Name:         Mark Simms
Profession:   Software Engineer
Organization: Hewlett-Packard Computer Peripherals Division
Unix-mail:    mark@hpcpbla.bri.hp.com
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Newsgroups: comp.periphs.scsi