Regarding the tape-drive based compression though, I think by default the drive 
does not compress (actually I'm fairly positive this is the case). The driver 
has to instruct the tape drive to enable hardware compression. In the case of 
solaris, this is done based on which device you are physically sending the 
backups to.

/dev/rmt/0 <-- The Tape Drive with no options.
/dev/rmt/0cbn <-- What you should be using. Enables HW compression and tells 
the tape drive to not automatically rewind.

The key being the 'c' in the device name. If you don't have that, then under 
Solaris the drive will not compress using its internal hardware compression.

Keep in mind though, if you are sending non-compressible binary files, 400GB is 
what you are going to get, regardless of if the client or drive is doing the 
compression. If you're sending highly compressible files (images for example) 
then you will get very good compression and your tape capacity will lean closer 
to the 800GB mark. If you're sending a mixture, then you will get somewhere 
between the two (depending on the type of files).

-----Original Message-----
From: veritas-bu-bounces AT mailman.eng.auburn DOT edu 
[mailto:veritas-bu-bounces AT mailman.eng.auburn DOT edu] On Behalf Of Justin 
Piszcz
Sent: Friday, November 21, 2008 10:48 AM
To: Haberl, Max
Cc: veritas-bu AT mailman.eng.auburn DOT edu
Subject: Re: [Veritas-bu] Reg: LTO3



On Fri, 21 Nov 2008, Haberl, Max wrote:

> Hi Justin,
>
> as far as i understand you enabled the compression in the policy right ?
No, compression is handled by the tape drive in hardware.  The compression
in the policy that you are referring to is whats referred to as client-side
compression, the data is compressed on the client before it is 'shipped' to
the netbackup media server, this can actual reduce actual compression rates
on the media itself because its sending a somewhat pre-compressed file to the
drive which cannot be compressed as well as it could be if it were a regular
file.  You would only want to use that option to save on bandwidth or if you
are backing up a host on a slow link.

> If yes, the benefit of the drive internal compression is around 0 cause you 
> still write a compressed tar file on tape.
See above.

>
> That means that the visible size of "bpmedialist -m <TAPE>" is already the 
> compressed size.
That command will show you how much is written on the tape once full then you
can calcuate the ratio based on an LTO-3 tape being 400GiB.

>
> Cheers
> Max


Justin.

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