On Wed, Oct 26, 2005 at 04:33:19PM -0700, Alan Jedlow wrote:
> Hi,
> 
> I know the "docs" say to turn hardware compression
> off before running amtapetype, unfortunately I've
> found no info on how to disable hw compression on a
> Dell PowerVault 122T LTO2 library.
> 
> How "bad" is this result ?
> 
> -----------------------------------------------------------------------
> -bash-3.00$ /usr/sbin/amtapetype -e 200g -f /dev/nst0 -t Ultrium2
> Writing 4096 Mbyte   compresseable data:  40 sec
> Writing 4096 Mbyte uncompresseable data:  350 sec
> WARNING: Tape drive has hardware compression enabled
> Estimated time to write 2 * 204800 Mbyte: 35000 sec = 9 h 43 min
> wrote 6422528 32Kb blocks in 98 files in 6420 seconds (short write)
> wrote 6422528 32Kb blocks in 196 files in 6719 seconds (short write)
> define tapetype Ultrium2 {
>    comment "just produced by tapetype prog (hardware compression on)"
>    length 200704 mbytes
>    filemark 0 kbytes
>    speed 31300 kps
> }
> -----------------------------------------------------------------------
> 

Most drives use a compression algorithm that expands,
rather than compressing, data that is random or nearly so.

LTO drives have the nearly unique feature for dealing with
this problem in that they sense the expansion potential and
turn off compression themselves.

So, if you use amanda with software compression and have
HW compression turned on, it is not the "terrible" situation 
that exists for other formats.  Amanda may underestimate
what it can fit on a tape, but it will unlikely overestimate.

The nice thing about that arrangement is should you have some
DLEs or complete hosts that you don't want to, or can't do
software compression the drive may still do a lot of compressing
for you.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
 4455 Province Line Road        (609) 252-0159
 Princeton, NJ  08540-4322      (609) 683-7220 (fax)