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Date: Wed, 29 Dec 2010 03:40:47 +0100
From: Spelic <spelic@shiftmail.org>
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Subject: [linux-lvm] pvmove painfully slow on parity RAID
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To: linux-lvm@redhat.com

Hello list

pvmove is painfully slow if the destination is on a 6-disks MD raid-5, 
it performs at 200-500Kbytes/sec! (kernel 2.6.36.2)
Same for lvconvert add mirror.

Instead, if the destination is on a 4 devices MD raid10near, it performs 
at 60MBytes/sec which is much more reasonable. (this is a 120-fold 
difference at least!)
Same for lvconvert add mirror.

How come such a difference?
Are you using barriers every tiny block of data maybe? (This could 
explain the slowness on parity raid)
If yes, could you use barriers (and hence checkpointing) every, like, 100MB?
(However please note that with lvconvert add mirror I also tried various 
--regionsize settings but they don't improve the speed much, i.e. +50% 
at most)

Thanks for any information