strange crypto speeds, sort of

[Jan Engelhardt]

Hi,


How come reading a file from a filesystem on an encrypted block device
is faster than reading the encrypted block device itself?

Linux home 2.6.23.17-rt #1 SMP PREEMPT RT 2007/10/26 14:17:15 
UTC i686 athlon i386 GNU/Linux
cryptsetup 1.0.5

/home # echo 3 >/proc/sys/vm/drop_caches
/home # dd_rescue foo /dev/null
...
Summary for foo -> /dev/null:
dd_rescue: (warning): /dev/null (120704.0k): Invalid argument!
dd_rescue: (info): ipos:    120704.0k, opos:    120704.0k, xferd:    120704.0k
                   errs:      0, errxfer:         0.0k, succxfer:    120704.0k
             +curr.rate:     8728kB/s, avg.rate:    13482kB/s, avg.load:  7.1%

/home # echo 3 >/proc/sys/vm/drop_caches
/home # dd_rescue /dev/mapper/home /dev/null
...
Summary for /dev/mapper/home -> /dev/null:
dd_rescue: (info): ipos:     21504.0k, opos:     21504.0k, xferd:     21504.0k
                   errs:      0, errxfer:         0.0k, succxfer:     21504.0k
             +curr.rate:     4291kB/s, avg.rate:     4291kB/s, avg.load:  5.8%
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