On Thu, Mar 06, 2008 at 12:04:39PM -0500, Paula Ehn wrote:
> We use Networker's compression on the client side so that we are
> transferring a smaller amount of
> data over our LAN to the backup server. To avoid compressing the data twice,
> we have compression turned
> off on our LTO-4 tape drives. For expample, we use /dev/rmt2hbn instead
> Problem is I'm seeing
> consistantly that only 792-794GB are being put onto each tape.

So you're losing a few GB from the advertised "up to 800GB".  I'm not
sure how much loss is normal.

> We use to use this same configuration when we used AIT-1 then later AIT-3
> tape drives and saw a wild spread in the amount of data being put onto a
> given tape. For AIT-3 it was anywhere from 120-320GB depending on how
> compressable the data was.

If you're disabling hardware compression, then there should be no spread
at all, right?  Anything over 100G means you're compressing data (or
that something is broken).

> For LTO-4 I'm expecting a similar spread but not less than the native
> capacity on the tape which is 800GB. I expect it to
> read better then 1TB on a tape at least.

I don't see how you'd get over 800 without hardware compression.

Where are your numbers coming from?  If you're looking at 'mminfo' and
other server-side info, they will ignore any client-side compression.
The server is handed the already-compressed stream and has no
information about how much client data is within.  It only records how
many bytes were sent to the tape.

-- 
Darren Dunham                                           ddunham AT taos DOT com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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