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From:   Overman_James
Sent:   Wednesday, July 16, 1997 1:30 PM
To:     [log in to unmask]
Subject:        Re: Problem with large DDS STOREs.

Curtis Larsen ([log in to unmask]) wrote:

: Overman_James <[log in to unmask]> wrote in article

: <SNIP>
: > Compress=High is best but one should turn off DDS hardware compression if
: your drive has it.
: <SNIP>

: On the average, is software compression better than the drive's hardware
: compression?

The compression algorithms are the same and so acheive the same degree of
compression.  However, TurboImage keeps compressing data into a block
until it is full, whereas the hardware compresses a given block without
merging more data into the block. (At least this is how I understand it).
Thus by our tests we get 3 to 5 % more on a tape with software compression
-- probably due to some sort of inter-block marks and fewer blocks with
software compression.  This is also why we use MAXTAPEBUF.

Note that the INTER option can speed up backups a bit 5-10%?? but as
it can complicate multi-tape and multidrive backup recovery (which tape
is the file on anyway??) and as the option had some problems in the past
we don't use it in the general case.   If anyone has more positive
comments on INTER I would be glad to hear them.

Thus, If you have adequate CPU power to perform the software compression
we think it is best.  The resultant tape can be read on any compatible
tape drive where the hardware compressed tapes can only be read on a
compressing drive.

--
 James Overman HP Support Technology Lab  Roseville, CA
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We have been using hardware compression on our two HP6400
DDS II drives for close to 2 1/2 years with INTER.  We have
had a couple of failures (discs) and had to rebuild the entire
system.  We have also done the usual restores due to testing
or Jim Trudeau being inadept.  We have had absolutly no
problems with INTER and MAXTAPEBUF.

Jim Trudeau
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