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user1091344
Asked: 2013-06-01 04:19:09 +0800 CST

RAID1 LVM, XEN and flashcache

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I am regarding to question: Improving IO with FlashCache

I have setup a RAID1 (/dev/md0) LVM (/dev/vg_xen) and executed following command:

flashcache_create -p thru flashcache0 /dev/sdc /dev/md0

Then I started a XEN PV guest, which uses /dev/vg_xen/vm0 as hard disk. After using it a while

cat /proc/flashcache/sdc\+md0/flashcache_stats

shows me

reads=0 writes=0

So, it does not work. What did I do wrong?

xen

1 Answers

  1. Best Answer

    You should make sure that your LVM is using the /dev/mapper/flashcache0 device as its physical volume instead of using /dev/md0 directly (in the latter case flashcache would not be involved in the I/O code path at all).

    See http://sf-alpha.bjgang.org/wordpress/2012/11/flashcache-initscripts/ for a handy script on how to set this up, if you have not done so yet. The basic outline would be:

    1. exclude /dev/md0 from being detected through LVM by adding filter = [ "r|/dev/md0|" ] and commenting out or removing filter = [ "a/.*/" ] (if present) from the devices{} section of your /etc/lvm/lvm.conf
    2. use the flashcache_create (or flashcache_load if using write-back caching mode) command to create your flashcache-backed device
    3. run vgscan to discover your volume group and activate it by issuing vgchange -ay vg_xen

    For details on flashcache configuration, refer to the admin guide which came with your flashcache download.

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