[dm-crypt] unusual writing speeds when benchmark speeds are high enough

Milan Broz gmazyland at gmail.com
Mon Mar 18 07:22:01 CET 2013

Please note that "benchmark" test is just appoximate.

If there is a problem with IO (storage - alignment etc)
always try to test with empty cipher - just use
-c null in luksFormat
(NOTE: this is plain copy, NO ENCRYPTION)
Available since http://code.google.com/p/cryptsetup/wiki/Cryptsetup143

This way you can check if the problem is in dmcrypt
overhead or some storage misconfiguration (data alignment etc).
(-c null will cause no kernel crypto is used at all, it is
plain data copy, just using the same dmcrypt IO path).

Also try direct io (add oflag=direct to dd) etc.
(This will eliminate page cache effect.)

In any case, try underlying device directly, then add another
layer, one per every step (LUKS, LVM, RAID, ...) so you can see where
is the problem. (See lsblk how the storage stack looks like.)


On 18.3.2013 0:19, .. ink .. wrote:
> I have asked the person for answers to the following questions,did i miss any useful question? I will report the answers when i get them
> what are your computer specifications?
> what kernel version are you running?
> are you on a 64 bit or 32 bit system?
> did you try the test on your hard drive with the OS or on a USB stick?
> did you run the test on an LVM volume or RAID device?
> A few more tests would be useful too,what do these commands give you?
> dd if=/dev/zero of=test1.img bs=10M count=1
> dd if=/dev/urandom of=test2.img bs=10M count=1
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