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[dm-devel] very slow dm-crypt write operation



Hi,

  I am seeing very slow dm-crypt writes on a machine where CPU, disk
speed and even dm-crypt reads are normal.
  It appears that kcryptd is scheduled insufficiently on the CPU(s)
when writing to dm-crypt devices. While reading, kcryptd is taking a
lot of CPU time, most of the time (top). But when writing, kcryptd is
taking a lot of pauses - I see intermittent activity where it uses the
CPU for a few seconds, then pauses for a long time, then it's up
again.
  Any idea what's going on?

  Writing a 1 GB file to dm-crypt takes 5 min (!):

time cp /tmp/1GB_File /mnt/dm-crypt/tmp/

real    5m21.123s
user    0m0.119s
sys     0m7.762s

  Reading the same 1 GB file back from dm-crypt takes only 1.5 min:

time cp /mnt/dm-crypt/tmp/1GB_File /tmp

real    1m30.147s
user    0m0.125s
sys     0m7.522s

  Copying the file without dm-crypt takes about 1 min:

time cp /tmp/1GB_File /tmp/Copy01

real    1m12.334s
user    0m0.090s
sys     0m5.856s

  This is Fedora 7, kernel 2.6.22.1-33.fc7 #1 SMP Mon Jul 23 17:33:07
EDT 2007 i686 i686 i386 GNU/Linux.
  Hardware is Intel 865G chipset, Pentium 4 CPU (HT), 2 GB RAM, ATA drive udma5.

  Thank you.


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