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



Are there are consequences to removing the metadata from most of the
physical volumes?

   You should be OK with one copy of the metadata, but of course that
means you can't later remove the PVs with metadata unless you first
put the metadata somewhere. Two copies the metadata provides redundancy,
more copies maintains redundancy even if some are lost or removed.

I've six, so I'd be adding a seventh and eighth (two
for redundancy, though the PVs are all built on RAID sets).

   If you have two redundant PVs or free space, one could move LVs in
order to empty an older PV, then recreate it with larger metadata. pvmove
can be used to move active LVs, or dd is much faster for inactive ones.

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?

Right.

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).

   Right.  As long as you do a vgcfgbackup, you're pretty safe.  I've
trashed things pretty badly before in various ways and vgcfgrestore has
been a great friend.  That said, it still wouldn't hurt to copy the LVs
to the new PV, then work on the old PV which is now redundant.  In which
case, you could then put a larger metadata area on the old PV.

What is the default metadatasize? Judging from lvm.conf, it may be 255.
255 Megabytes?

  I believe it defaults to 255 sectors, roughly 128KiB.


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

128KiB will cover something on the order of 20 LVs. (Very roughly speaking). If you're using PVs of a terabyte or more, you could probably easily spare 16MB, which would be 50 times the default. That's what we use because we don't ever want to have to worry about it again. 16MB will allow for roughly 1,000 LVs.
pvmetadatasize = 32768

Hmm, at the time I choose 16MB I thought it would be more than enough, but we're already at 171 LVs, so I guess I'll use 64MB for metadata on our new PVs.
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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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