>What is the "sample" size for determining duplication of data?  If it's
>64MB or 256MB blocks, am I going to get many hits, as the same data may
not
>be blocked the same way each time?

The sample size is much smaller than that, as small as 16K-256K.  It's
all over the board, though.

>If I de-dupe, there's got to be a catalog/database.  How is it
protected?
>I assume that's vendor specific.  

You betcha, and that's a question you should ask that vendor.

>Does de-dupe work with encrypted data?  Compression normally doesn't,
so I
>suspect the answer would be no, but someone may have come up with a
system
>to do it.

You cannot encrypt the data before it goes to the VTL and expect it to
de-dupe it.  But you can encrypt when copying from the VTL to tape, or
you can put an encryption box behind the VTL head but in front of the
disk and you're OK (although I don't see the point).

>And does anyone have any real world, TSM based, numbers on the space
>savings they can share?  

I've seen 10:1 - 15:1 in TSM environments, higher ratios in other
products due to full backups.  But since de-dupe still finds duplicated
blocks in files that are modified repeatedly and files that are simply
stored in multiple places, and since TSM environments still do full/incs
on databases, they're still plenty to get rid of.