Tape compression, and, 3581 vs 3590

Every night, after the backup we perform the following, and have it 
automatically emailed to us:
     PRTERRLOG TYPE(*VOLSTAT) +
               VOLTYPE(3590) +
               VOLSTAT(*DLT)

A sample report looks like:

Title . . . . . . . . . . :  Volume Statistics Report
System type . . . . . . . :  9406
System model  . . . . . . :  840
System release  . . . . . :  V5R2M0
System name . . . . . . . :  GDIHQ
...
Volume      ---Temporary Errors---   --------K Bytes--------
ID                Read       Write         Read      Written

3MON23               0           5           24     50859901
3MON33               0           3           24     51168097
3MON13               0           1           29     55707376
3MON63               0           1           58    122769204
3MON43               0           0           29     62088769
3MON53               0           0           13     25839073
4MON1                0           0           56    115762000
4MON2                0           0           53    115593409
4MON3                0           0           52    113884949
4MON4                0           0           52    111955033
4MON5                0           0            9     16954814
6MON1                0           0           57    116638708
6MON2                0           0           56    122654197
6MON3                0           0           54    117434773
6MON4                0           0           37     60546789

3 is TAP03, a 3590.
4 is TAP04, a 3581.
6 is TAP06, a 3581.
3MON13 through 3MON53 are used in
  SAV ... 
  of Domino directories, no compression specified.
3MON63 is used in
  SAVLIB ... DTACPR(*DEV)
4MON. and 6MON. are used in 
  SAVOBJ ... , no compression specified
  out of library PCBACKUP, (Tivoli Storage Manager)
  I doubt that we'd get much compression.  TSM already compresses data 
pretty good.  We are NOT using TSM to put the data to tape.  We use TSM to 
save the PC's to disk and then use SAVOBJ to save the TSM objects to tape. 
 We are not using the PASE version of TSM yet.  (As you can tell by the 
use of a library instead of a directory.)

Question 1:
Why is the SAVLIB on 3MON63 writing twice the bytes per tape as the rest 
of the 3MON..?  Is it because the SAV of the Domino directories do not 
specify any compression?

Question 2:
Shouldn't the 3581 be writing a lot more bytes that the 3590?

Rob Berendt
-- 
Group Dekko Services, LLC
Dept 01.073
PO Box 2000
Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com