On Tue, 3 Sep 2002, WEAVER, Simon wrote:

> I got a small question about Compression! In Netbackup Policy
> Management, you can define "Software" Compression by choosing the Tick
> box on the Attributes Tab.
>
> However, I was advised that my ADIC Scalar 100 comes with Hardware
> Compression. I am trying to identify how I can determine if Hardware
> Compression is being used. My reason for asking this is because we
> done a 109GB Backup which spread over 2 tapes. We use LTO 200GB
> Compressed Tapes. Surely this should have gone onto the same tape?
>
> Does anyone know the best way to check the ADIC for compression?

Your results with any compression, hardware or software, depend greatly
on the compressibility of your data.  If you're trying to backup
compressed image data or archived .zip or .gz files, you'll see no gain,
or very little gain, in trying to compress it again. If you're backing
up things like databases, you'll often see 8:1 or even better compression.

I'm not sure how it works under Windows NT, but under Unix, the hardware
compression of the IBM LTO drives used by ADIC is controlled by the
device drivers for those drives.  Someone else here, or probably ADIC if
you ask them, can give you better information about checking for
hardware compression under Windows.

I just wanted to point out that 109GB being split across two tapes is
entirely possible if the source data is already well compressed.

The hardware compression numbers used by /all/ tape drive manufacturers
are based on what you can expect for an average mix of data.  Most drive
makers tend to list this as 2:1, though some are bold enough to list
3:1.  All of those numbers are a load of crap and are dependant on your
specific application.  You should always check the results based on
running some of your own test data through a drive.  Or in my case,
because I'm dealing with lots of compressed image data, I just always
assume I'll get 1:1 compression.  The additional compression I do see
I count as a nice bonus.

-brian.