Bug#637406: linux-2.6: Slow lvm performance with raid1 and 1 missing device

[Michael Moritz <michael@gn.apc.org>]

Package: linux-2.6
Severity: important

I've set up a raid1 device with one missing device and created a lvm logical volume on it (pvcreate, vgcreate, lvcreate). Disk IO is significantly slower than when I do the same set-up using the onde device directly and no raid1. I've tested this with different hard drives and on different machines. According to 'top' the [kdmflush] hogs the CPU at 99-100% during any access to the filesystem on logical volume. I've also tested this with an earlier kernel (2.6.26 lenny) and got the same problem. According to iostat when using the device disk utilisation of the underlying device is constantly 100%.
   
Thanks,

Michael
-- System Information:
Debian Release: 6.0.2
  APT prefers stable
  APT policy: (900, 'stable'), (500, 'stable-updates'), (100, 'oldstable')
Architecture: i386 (i686)

Kernel: Linux 2.6.32-5-686 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash