[Bug 875733] [NEW] disk cache is slow on average with dm-crypt

[Dmole]

Public bug reported:

I would expect the cached read speeds to be the same as the cache is above the encryption  device mapper layer.
Using dm-crypt read speeds are inconsistent averaging 380 MB/s, not using  dm-crypt read speeds are consistently 1200 MB/s.
I used dd to test at least 12 times:
dd if=./test.file of=/dev/null bs=1M count=1024

lsb_release -rd:
Description:	Ubuntu 10.04.3 LTS
Release:	10.04

apt-cache policy cryptsetup:
cryptsetup:
  Installed: 2:1.1.0~rc2-1ubuntu13
  Candidate: 2:1.1.0~rc2-1ubuntu13
  Version table:
 *** 2:1.1.0~rc2-1ubuntu13 0
        500 http://us.archive.ubuntu.com/ubuntu/ lucid/main Packages
        100 /var/lib/dpkg/status

** Affects: cryptsetup (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/875733

Title:
  disk cache is slow on average with dm-crypt

Status in “cryptsetup” package in Ubuntu:
  New

Bug description:
  I would expect the cached read speeds to be the same as the cache is above the encryption  device mapper layer.
  Using dm-crypt read speeds are inconsistent averaging 380 MB/s, not using  dm-crypt read speeds are consistently 1200 MB/s.
  I used dd to test at least 12 times:
  dd if=./test.file of=/dev/null bs=1M count=1024

  lsb_release -rd:
  Description:	Ubuntu 10.04.3 LTS
  Release:	10.04

  apt-cache policy cryptsetup:
  cryptsetup:
    Installed: 2:1.1.0~rc2-1ubuntu13
    Candidate: 2:1.1.0~rc2-1ubuntu13
    Version table:
   *** 2:1.1.0~rc2-1ubuntu13 0
          500 http://us.archive.ubuntu.com/ubuntu/ lucid/main Packages
          100 /var/lib/dpkg/status

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