[dm-crypt] FSB saturation?

Kyle Rose krose at krose.org
Sat May 14 16:36:31 CEST 2011

For the past week I've been trying to figure out why my
dm-crypt-on-RAID6 performance is so terrible. To wit:

root at yupa:/source# hdparm -t /dev/md1

/dev/md1: <---the RAID 6 block device
 Timing buffered disk reads: 1262 MB in  3.00 seconds = 420.19 MB/sec

root at yupa:/source# hdparm -t /dev/mapper/yupa

/dev/mapper/yupa: <---the dm-crypt device with /dev/md1 as the underlying store
 Timing buffered disk reads: 370 MB in  3.00 seconds = 123.26 MB/sec

The machine (a Core 2 Quad) is not anywhere near CPU bound in either
case (even with all the bulk encryption), and is obviously not disk
bound in the latter. Besides having used linux-2.6.38 and
cryptsetup-1.1.3 to build the system, post-hoc analysis of iostat
indicates that all layers are properly aligned. My working theory is
FSB contention caused by lots of memory copies during the process of
moving data through each layer. Has anyone else run into this?

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