Detailed info about backups

[Paul Furness <paul.furness@vil.ite.mee.com>]

Hello.

Can anyone give me some pointers for comprehensive information about
tape backups?

A brief bit of background:

I have linux servers running mostly FC2, some RH9 and a couple of RH73.
One of the servers has some autochangers attached - a DLT, an LT01 and
an LTO2. I have scripts which automate which tape to load based on the
day and what's being backed up. Control of the changers is done using
mtx, control of the drives is done with mt, and the backups are done
using xfsdump and dump from remote machines.

The main problem I have is that I seem to be running out of space on the
tape a long time before I've stored anywhere near as much data on there
as the tape should hold - for instance, I'm trying to back up about 150G
on an LTO2 tape which has native capacity of 200G, and it's failing with
a message suggesting the tape is full.

There are a couple of specific things I'm trying to find out; maybe
someone knows the answers to these and can answer me direct :o)    

What does "block size" mean on a tape (DLT and LTO). Is it the same as
old disks, where each file fills up at least one block, even if it's
much smaller than a block? If I make this smaller, will more small files
fit on the tape?

How do I get the tape drive to definitely use it's internal compression
hardware? I don't think it's working and I need it to be. How can I
check this from linux? Some of the auto-changers have displays on the
front that specify whether compression is being used, but others don't.

Is there any way of finding out how much of the tape has been used by a
backup - in terms of actual tape, not in terms of how much data there
was in the backup? I really want to be able to work out how much space
is left on the tape.
 
Answers to any and all of the above questions very much appreciated.

Thanks.

Paul.