At one point in the evolution of out TSM servers we tried to have separate
tape drives set
up for compression and no compression.  We quickly abandoned this.  We have
a real mixture
of data that is partly already compressed on the client (oracle backups)
and data that isn't compressed
already, as well as some clients that perform TSM client compression.  It
was too comfusing and complicated!

We punted . . . . all tape drives are set for compression.

Yes, we have the tape drives compress a lot of data that is already
compressed.  We accept
this to make our TSM server administration easier.

But . . . .I've also read a article about a IBM compression chip they
manufacture and used in Sony AIT drives.  The chip was smart enough
to stop  compression if the output object was growing in size.  I would not
assume that a tape drive can do this, but I wouldn't assume that it doesn't
do it either.

Rick




                      Richard Sims
                      <rbs AT BU DOT EDU>             To:       ADSM-L AT 
VM.MARIST DOT EDU
                      Sent by: "ADSM:          cc:
                      Dist Stor                Subject:  Re: Tape Drive 
Compression over TSM Compression
                      Manager"
                      <[email protected]
                      .EDU>


                      08/09/2004 02:54
                      PM
                      Please respond to
                      "ADSM: Dist Stor
                      Manager"






>what are the implications on doing this?
>I saw on the Redbooks that this is not recommended!

Data compression is not always benign...  Trying to compress
already-compressed data can actually cause it to increase in size.
Ideally, you only want to compress data once.  In reality, it may
be beyond your control, as some facility on the client system may
have compressed it before TSM or the tape drive get it.  In
general, avoid knowingly causing multiple compression operations:
have either the TSM client do it, or the tape drive, but not both.

  Richard Sims




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