[CentOS] pvmove speed

Wed Feb 13 18:26:23 UTC 2008
Ross S. W. Walker <rwalker at medallion.com>

Joseph L. Casale wrote:
> 
> Are there any ways to improve/manage the speed of pvmove? Man 
> doesn't show any documented switches for priority scheduling.
> Iostat shows the system way underutilized even though the lv 
> whose pe's are being migrated is continuously being written 
> (slowly) to.

I don't believe pvmove actually does any of the lifting. Pvmove
merely creates a mirrored pv area in dev-mapper and then hangs
around monitoring it's progress until the mirror is sync'd up
then it throws a couple of barriers and removes the original
pv from the mirror leaving the new pv as the new location for
the data.

That is how the move continues through reboots. All lifting
is actually done in dev-mapper and it's state is preserved
there. On restart LVM will read it's meta-data to determine
if there is a pvmove in progress and then spawn a pvmove to
wait for it to complete so it can remove the mirror.

Any slowness is due to disk io errors and retries being
thrown around.

You should really run LVM on top of a RAID1, software or
hardware makes no difference, but LVM is more to storage
management then fault tolerance and redundancy.

-Ross

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