inconsistent performance for 2 partitions on the same disk

[Stefan Monnier]

I noticed that my backups always report a discrepancy in the dump-speed
of my / and /home partitions:

HOSTNAME  DISK      L  ORIG-KB   OUT-KB COMP%  MMM:SS   KB/s  MMM:SS   KB/s
--------------------- -------------------------------------- --------------
tequila   /         0   819350   313760  38.3   17:44  295.0   10:44  487.6
tequila   /home     0   900490   307008  34.1   31:23  163.1   10:32  485.9

As you can see, the KB/s are widely different (they are much higher now that I
switch to 2.1, but the difference was similar on 2.0).
The / and /home partitions are on the same SCSI disk, using the ncr53c8xx
driver.
The difference is not completely unexplained since the backup system I use
always dumps to disk before streaming to tape and the temporary hold area
is (in my case) in /home.  Now why should dumping /home onto itself be slower
than dumping / onto /home ? (yes, I have been careful to exclude the hold area
from the dump).

Is there a chance that I'm hitting some strange `per partition queue' or
somesuch ?

	Stefan

PS: if you don't have any idea, it's no problem:  I can think of a few
    other explanations for this discrepancy (even though they seem less
    likely because of the repeatability of the problem)