[dm-crypt] Poor performane (idle cpu)

[Jakob Sandgren <jakob@southpole.se>]

Hi,

I'm using dm-crypt for several mappings with a hardware raid backend. 
Using a raw read from the raid device (e.g sda) gives ~250MB/s

But when I read from an encrypted mapping, I just get ~70MB/s. That
should be fine if I at least have the kcryptd process using a core at
100%, but that is not the case. Three of my four cores is 99% idle and
one core is 50% idle (aprox.).

I have recently upgraded my hardware from an older quadcore system
(AMD) to a new Core I7 (860) and expected improved performance and
when I did not get that, then did I do some more investegation and
found out above. I have also read posts from others having the same
problems, but no explanation.

Anyone who has an explanation for this or a hint if one could
"tune" anything to get better performance? 

Output from "cryptsetup status":

/dev/mapper//dev/mapper/shared1 is active:
  cipher:  aes-cbc-essiv:sha256
  keysize: 128 bits
  device:  /dev/mapper/areca_0_1-shared1_raw
  offset:  1032 sectors
  size:    3145726968 sectors
  mode:    read/write


Best Regards,
Jakob


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