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Date: Thu, 18 Nov 2010 21:32:58 +0000
From: Alasdair G Kergon <agk@redhat.com>
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Subject: Re: [linux-lvm] Move pvmove questions Was: Need help with
	a	particular use-case for pvmove.
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To: LVM general discussion and development <linux-lvm@redhat.com>

On Thu, Nov 18, 2010 at 03:40:25PM -0500, Stirling Westrup wrote:
> It mentions checkpointing, but gives no further information. It
> certainly doesn't say anything about how often they are done, or under
> what circumstances. 

It's in there, but the man page would indeed benefit from more repetition
and examples.  Patches welcome!

       Every logical volume in the volume group is searched for *contiguous
       data* that need moving according to the  command  line  arguments.   For
       each  piece  of  data  found,  a new *segment* is added to the end of the
       pvmove LV.  This segment takes the form of a *temporary mirror*  to  copy
       the data from the original location to a newly-allocated location. 

       A daemon repeatedly checks progress at the specified time interval.
       When it detects that the first *temporary mirror* is in-sync,  it  breaks
       that  mirror  so that only the new location for that data gets used and
       writes a *checkpoint* into the volume group metadata on  disk. 

So checkpoint at first daemon check interval after temp mirror in-sync, and
one temp mirror per item of contiguous data to be moved.

Alasdair