Compressed data backup

[Kapil Jha]

Gurus,
I have a small doubt about compressed data backup.
How do we configure compressed data backup for a tape(tape says 36GB normal and 72GB compressed).
SO where we would define that compressed backup would be taken.
BR,
Kapil

[Steven Schweda]

It might help to know the model of the tape
drive. (What's rated 36/72GB?)

"man tape":

[...]
Device Special Files and Tape Density

Using a particular device special file with a certain tape format will
indicate what density code, compression, capacity and I/O is supported. [...]

[Kapil Jha]

"man tape" clears a lot.
so what I have understood is

"Previous Format in /dev New Formats in /dev/tape and /dev/ntape
_______________________________________________________________________
rmt?a tape?_d3
rmt?l tape?_d0 (d0 always maps to the low density)
rmt?m tape?_d2
rmt?h tape?_d1 (d1 always maps to the high density)
_______________________________________________________________________

specifying d1(what about d[2-6]) would enable high density backup.
But where do we specify that backup running is goin to be compressed or normal.
Is it like
using tape?_d0 would result in normal backup
and
using tape?_d1 result in compressed backup.

> hwmgr v d -id 251
HWID: Device Name Mfg Model Location
------------------------------------------------------------------------------
251: /dev/ntape/tape41 HP Ultrium 2-SCSI bus-4-targ-6-lun-1

# hwmgr -get attrib -id 251
......
current_density = 66 (what does this signify)
....

Thanks a lot for your help.

PS. I could not find tape type as my frnd facing this problem has disappeared :)

BR,
Kapil

[Steven Schweda]

> Is it like [...]

It may depend on what's in /etc/ddr.dbase for
your tape drive. On my system, all I see
for Ultrium is:

[...]
Name = "HP" "Ultrium"
[...]
#
# Ultrium density parameters
#
DENSITY:
#
DensityNumber = 0,2,4,6
DensityCode = default
CompressionCode = 0x0
Buffered = 0x1

DENSITY:
#
DensityNumber = 1,3,5,7
DensityCode = default
CompressionCode = 0x1
Buffered = 0x1
[...]

from which, I'd guess that tape?_dX (X = 1,
3, 5, 7) would use compression.

If you got an update to /etc/ddr.dbase for
your fancy new tape drive, then you may have
stuff different from mine.

[Kapil Jha]

Thanks a lot u r BOND :)
well one more question what is the difference between 0 2 4 6
and 1 3 5 7.
Is is for % compression or what???
BR,
Kapil

[Steven Schweda]

> Is is for % compression or what???

So far as I know, compression is binary --
off ("CompressionCode = 0x0") or on
("CompressionCode = 0x1"). I assume that
anything else in the DensityNumber would
select a density, but I don't know which one
means what (if this drive even has more than
one density). It's also possible that
there's no real difference among 1, 3, 5, 7.
If there is some kind of density indication
on the front of the drive (LEDs, fancy LCD),
then some experimentation might be
educational.

[DCBrown]

tape density '66'

in ddr.dbase near the top is a tape density table. The '66' (x42) is an index into that table:

scsi_tape_density[0x42] = "186000_bpi" 186000 0


When compression is turned on, this means h/w compression on the device is turned on. You can often confirm this during operations by looking at the front panel lights of the tape drive - compression light should be lit.