2008-03-24 22:19:28

by Jan Engelhardt

[permalink] [raw]
Subject: strange crypto speeds, sort of


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

Linux home #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%