[CentOS] Best way to extend pv partition for LVM

Sat Apr 2 17:02:19 UTC 2011
Jay Leafey <jay.leafey at mindless.com>

Kai Schaetzl wrote:
> I've replaced disks in a hardware RAID 1 with larger disks and enlarged 
> the array. Now I have to find a way to tell LVM about the extra space.
> It seems there are two ways:
> 1. delete partition with fdisk and recreate a larger one. This is 
> obviously a bit tricky if you do not want to lose data, I haven't 
> investigated further yet.
> 2. create another partition on the disk, pvcreate another pv and then add 
> it to the existing volume group with vgextend
> 3. a possible third way: increase the partition size. According to Google 
> most if not all disk tools want to resize a file system as well and since 
> there is no file system they will fail. I'm not sure about the status with 
> this for the tools that come with CentOS (fdisk, parted, other?)
> No. 2 seems to be the easy way. Any objections?
> One I thought of:
> What does happen when I use No. 2 and I add new lvs? Can it happen that 
> new lvs get "spanned" over both pvs or can I assure that a pv gets created 
> using only one of the pvs? (I would prefer the latter, it doesn't matter 
> if I use a few MB because of the ineffectiveness of allocation.)
> Thanks for recommendations.
> Kai


I ran into the same circumstances a while back and, after a lot of 
consideration and testing, I chose door #2.  It's the most expedient way 
to do it if you have no other resources available, but it does have some 
inefficiencies involved in it.

You COULD use option #1, but it requires some additional resources and a 
LOT of shuffling.  Specifically:

- add an extra disk to the volume group (pvcreate, vgextend)
- move the extents off of the "old" PVs onto the new PV using pvmove
- drop the PVs from the old disks out of the VG
- delete the PVs from the old disks
- repartitioned the old disks
- created PVs on the old disks
- added the new/old PVs to the VG
- move the extents from the temporary PV to the new/old PVs
- remove the temporary disk from the VG, delete the temporary PV

WAY too much shuffling and moving parts to suit me.  The system never 
has to shut down, but performance can truly go into the dumper while 
pvmove is shuffling bits.

I was originally thinking option #3 would be the "best" (i.e. most 
efficient) way, but on a test system I tried several times to extend the 
partitions to include the extra space and failed miserably.  Like you, I 
got stalled at the point of trying to extend the partitions.  parted 
seems to refuse to do so unless there is a supported filesystem on the 
partition, which does not seem to include LVM.

If anybody has some hints about how to work around parted's reluctance 
to merely extend an arbitrary partition, I'd really like to know!  I 
know I could use fdisk to delete the old partition and recreated it at a 
larger size, but that scares the bejesus out of my timid soul, having 
had power failures during critical operations on more than one occasion.
Jay Leafey - jay.leafey at mindless.com
Memphis, TN
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/x-pkcs7-signature
Size: 5529 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://lists.centos.org/pipermail/centos/attachments/20110402/a8cc7537/attachment-0003.bin>