newbie: Speed of loop-aes

Hi List,

I did some performance tests using the benchmark bonnie++ to evaluate
different disk encryption possibilities. The results regarding loop-aes
are a bit strange for my understanding. I ran the tests on a notebook
with Debian Etch, 1GB RAM and a Linux 2.6.22.2 Kernel.

Bonnie was startet with the following parameters:
bonnie++ -b -u root -s 4000

As a result loop-aes has a sequential input (block) of 29127 K/sec where
the sequential input with no encryption is 25123 K/sec. The same with
sequential output (block) (loop-aes: 27633 K/sec and no encryption:
24267 K/sec).

What I don't understand is how encrypted reads and writes per block
could be faster as plaintext reads / writes? Any comments are welcome!

Kind regards,
Markus

P.S.: The bonnie++ output:
LOOP-AES:
 ------Sequential Output------ --Sequential Input- --Random-
 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
4000M 18554  59 27633   7 13696   3 18922  77 29127   4 102.3   0

NO ENCRYPTION:
 ------Sequential Output------ --Sequential Input- --Random-
 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
4000M 24085  77 24267   6 11978   3 24825  75 25123   3 103.9   0


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