[linux-lvm] performance of dm-crypt devices?

[Tomasz Chmielewski <mangoo@wpkg.org>]

I just set up an encrypted LUKS device using the information on 
http://www.saout.de/tikiwiki/tiki-index.php?page=EncryptedDeviceUsingLUKS

There was a suggestion there, which prompted me to check the performance:

   If you wish, use /sbin/hdparm to benchmark. However my benchmarks on
   an AMD Athlon 3200 indicate no great difference between an encrypted
   and a normal unencrypted partition.


First, I set read-ahead to the same value on both devices (original LVM 
device, and the crypted one):

# blockdev --setra 16384 /dev/mapper/crypttest
# blockdev --setra 16384 /dev/mapper/san1-test


Next, hdparm test:

# hdparm -t mapper/crypttest mapper/san1-test

mapper/crypttest:
  Timing buffered disk reads:  116 MB in  3.01 seconds =  38.54 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate 
ioctl for device

/dev/san1/file1-swap:
  Timing buffered disk reads:  304 MB in  3.12 seconds =  97.46 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate 
ioctl for device


So, this quick test suggests that with a crypted device, I get about 40% 
performance of the original LVM volume?


My setup is hardware RAID-10, and a dual core 3 GHz Xeon.

Is it normal? I would say yes, as both cores use 100% CPU when I do 
intensive reads from an encrypted volume.
However, this seems to contradict with "However my benchmarks on an AMD 
Athlon 3200 indicate no great difference between an encrypted and a 
normal unencrypted partition".


I used aes-cbc-essiv:sha256 cipher.

Perhaps, I should use something lighter?



-- 
Tomasz Chmielewski
http://wpkg.org