To add to this, what I do is to compute an average compression ratio for my
volume pools based on how much data was written to a "full" tape and
dividing that by the stated tape capacity.  With that snippet you can do
some reasonable predicting on how much more data can still be written to the
tape.

-----Original Message-----
From: Darren Dunham [mailto:ddunham AT taos DOT com]
Sent: Wednesday, June 05, 2002 12:01 PM
To: Veritas-bu AT mailman.eng.auburn DOT edu
Subject: Re: [Veritas-bu] Tape space


> 
> Is there an easy way to determine how much actual
> space a backup is using on a DLT tape. Since Netbackup
> reports space used based on uncompressed sizes. 

No, not easily. 

The main problem is that you can't easily tell how well the compression
is working.  For the hacker, there is apparantly a code register on the
device that can be queried via a scsi command.  That register contains
the average compression over the tape.  A little math and you'd get
pretty close.  I don't know of any utilities that are written to query
it, and my programming skills have not improved over the past few years
for me to try.

A smaller problem is that the drive checks blocks during writing, and
can re-record them later in the tape if they are marginal.  This unseen
expansion can use up extra tape, even if you're writing uncompressed.
It's unlikely to add up to much total space on a tape, but it does mean
that you may not necessarily be able to fit one tapes worth of data onto
a second tape.

Most programs and admins just give up and use the total space written on
a tape as only a guideline of how much more is likely to to fit.


-- 
Darren Dunham                                           ddunham AT taos DOT com
Unix System Administrator                    Taos - The SysAdmin Company
Got some Dr Pepper?                           San Francisco, CA bay area
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