[linux-lvm] mirror_region_size default: 4096 six times faster than 512?

[Ray Morris <support bettercgi com>]

Moving several terabytes of data, I was again struck by the very 
large difference in performance between copying data in small chunks
for large chunks. After seeing larger block sizes make dd eight times
faster, I tried increasing mirror_region_size in lvm.conf:

pvmove with default mirror_region_size of 512: ~4-6 MB/s
pvmove with increased mirror_region_size of 4096: ~22-32 MB/s

Although this was somewhat unscientific testing, I'm quite curious 
what results others get on their hardware. The effect was most 
pronounced for me on a system with a 9550SX-12. I'll do some testing 
on a 8888ELP next week.

If 4096 is OFTEN several times faster than 512, would it make sense 
to consider canging the default?
-- 
Ray Morris
support bettercgi com

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