[linux-lvm] Solving the "metadata too large for circular buffer" condition

Ray Morris support at bettercgi.com
Wed Nov 24 23:07:57 UTC 2010

   Taking another look, my estimate of required metadatasize
might be off by a factor of five or so.  128K might be sufficient
for 100 LVs, depending on fragmentation and such.

   Still, the history of computer science is largely a story
of problems caused by people thinking they'd allocated plenty
of space.  Today you can't fdisk a drive or array larger than
2TB because someone thought 32 bits would plenty.  Probably
best to allocate 100 times as much as you think you'll ever
need - a few MB of disk space is cheap.
Ray Morris
support at bettercgi.com

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On 11/24/2010 02:28:11 PM, Andrew Gideon wrote:
> We've just hit this error, and it is blocking any expansion of  
> existing
> or creation of new volumes.
> We found:
> http://readlist.com/lists/redhat.com/linux-lvm/0/2839.html
> which appears to describe a solution.  I'm doing some reading, and  
> I've
> set up a test environment to try things out (before doing anything  
> risky
> to production).  But I'm hoping a post here can save some time (and
> angst).
> First: The referenced thread is two years old.  I don't suppose  
> there's a
> better way to solve this problem today?
> Assuming not...
> I'm not sure how metadata is stored.  It seems like, by default, it is
> duplicated on each PV.  I'm guessing this because one can't just add  
> new
> PVs (with larger metadatasize values), but one must also remove the  
> old
> metadata.  Is that right?
> Are there are consequences to removing the metadata from most of the
> physical volumes?  I've six, so I'd be adding a seventh and eighth  
> (two
> for redundancy, though the PVs are all built on RAID sets).
> The "pvcreate --restorefile ... --uuid ... --metadatacopies 0" command
> would be executed on the existing 6 physical volumes?  No data would  
> be
> lost?  I want to be *very* sure of this (so I'm not trashing an  
> existing
> PV).
> What is the default metadatasize?  Judging from lvm.conf, it may be  
> 255.
> 255 Megabytes?  Is there some way to guestimate how much space I  
> should
> expect to be using?  I thought perhaps pvdata would help, but this is
> apparently LVMv1 only.
> [Unfortunately, 'lvm dumpconfig' isn't listing any data in the  
> metadata
> block.]
> There's also mention in the cited thread of reducing fragmentation  
> using
> pvmove.  How would that work?  From what I can see, pvmove will move
> segments.  But even if two segments are moved from dislocated  
> locations
> to immediately adjacent locations, I see nothing which says that these
> two segments would be combined into a single segment.  So I'm not  
> clear
> how fragmentation can be reduced.
> Finally, there was mention of changing lvm.conf - presumably,
> metadata.dirs - to help make more space.  Once lvm.conf is changed,  
> how
> is that change made live?  Is a complete reboot required, or is there  
> a
> quicker way?
> Thanks for any and all help...
> 	Andrew
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