As usual, "better" depends on your point of view.

If you do the compression in the software, the client machine has to spend
processor cycles doing the compression, and the backup can actually take
longer in terms of elapsed time.  Restores can take longer as well.  But,
you send a LOT less data across your network.  Whether it's good for you,
depends on your client and your network.

If your client machine has a fast processor, and it is backing up in the
middle of the night and has nothing else to do during that time, software
compression may work well for you, as long as your restores still run fast
enough to meet your recovery requirements.

Doing the compression in the hardware has the advantage that the compression
is done outboard, via the processors imbedded in the tape unit, so there is
no client overhead cost to using it.  But, you are sending a lot more data
across your network from the client to the server.

And there are cases where (as mentioned on the list) if the data is already
compressed, it can actually grow a bit when compressed the second time in
the hardware, although I have seen no rule of thumb for how much (and I
think it will vary depending on your type of hardware).

In general, I start out with software COMPRESSION ON as the default for most
clients that don't have stringent recovery time requirements, and turn it
off selectively for cases where COMPRESSION OFF seems to be a better idea.
Some cases where you can know for sure that software compression is not a
good choice:

*       Mail server machines which have their mail data bases already
compressed.  In this case you know that the backup client software will be
spending a lot of time trying to compress something that won't compress
significantly.

*       Some web server machines where the majority of files are .JIF and
.TIF files.  These files are also already compressed, and the client
software will be spending a lot of time to accomplish nothing.

 ************************************************************************
Wanda Prather
The Johns Hopkins Applied Physics Lab
443-778-8769
wanda_prather AT jhuapl DOT edu

"Intelligence has much less practical application than you'd think" -
Scott Adams/Dilbert
************************************************************************



> -----Original Message-----
> From: Randy Tackett [SMTP:rltackett AT AEP DOT COM]
> Sent: Friday, October 01, 1999 9:23 AM
> To:   ADSM-L AT VM.MARIST DOT EDU
> Subject:      Hardware vs. Software Compression
>
> Which is better? Hardware or software compression? What are the benefits
> of
> each? What are the shortcomings of each?