I belive the answer to your question, is that it depends on your tape drive 
hardware/firmware. 
On the old 3480 drives, for example, recompressing already-compressed data 
would cause it to expand.

But I thought one of the features of the TS1120's and higher, is that the drive 
is smarter and if data starts expanding it stops the compression and writes the 
data as is.

So I'm VERY curious to see you are getting those stats on 3592 media.

W 

________________________________________
From: ADSM: Dist Stor Manager [ADSM-L AT VM.MARIST DOT EDU] on behalf of Scott 
McCambly [mccambly AT UNOPSYS DOT COM]
Sent: Thursday, December 09, 2010 11:26 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] ULTRIUM4C but not 1.6TB

This is related to a similar question I have been investigating on 3592
media:   Why would you ever get less than the native capacity of a
tape?

Does writing compressed data to tape with hardware compression enabled
result in the data expanding?

I always assumed that the hardware compression mechanism would have
something equivalent to "CompressAlways=NO" and detect already
compressed data in its input buffer, however we backup a number of
already compressed file formats and often see FULL tapes with estimated
capacities from 5 to 20% less than the stated native (uncompressed)
capacity.

Can anyone confirm this?

Thanks.

On Thu, 9 Dec 2010 16:30:18  0100, Hans Christian Riksheim  wrote:
If a LTO4 tape holds more than 800GB compression is on.
  >
  > At our place, file data only has a 1.1:1 compression while Oracle data and
  > mail is 2.5:1. Overall it is 2:1.
  >
  > Hans Chr.
  >
  >
  > On Thu, Dec 9, 2010 at 9:18 AM, Mehdi Salehi <ezzobad AT gmail DOT com> 
wrote:
  >
  > > Hi,
  > > Drives are LTO4, devclass is ULTRIUM4C, but all "Full" volumes
are between
  > > 800GB to 1TB. Is it normal? It seems TSM does not use compression.
  > >
  > > Mehdi
  > >
  >
  >