How to resize a Volme Group ?

marozsas · 03-17-2009, 08:56 PM

Dear Gurus !

I installed F10 in a new 500G disk using the defaults. Bad move

Now I have a big partition with a big Volume Group on it, with a big logical group on it with a big filesystem on it. Bad, bad move, indeed...

Using a Live CD, I manage to resize the filesystem using resize2fs and then, using lvreduce, I shrink the Logical Volume too.

Next step: Resize the Volume Group and after that, reduce the LVM(8e) partition and free space on disk for other partitions.

The big question is: How to reduce a Volume Group ? I already extended a VG adding another PV to it, but this VG has only one PV as big the underlying partition.

I don want't to start over again. I already put my system up to date adn installed a lot of additional software online.

Please, tell me dear Gurus there are light in the darkness ! Enlight me! Show me the TRUE !

cheers !

PS: BTW, I tried "pvresize --setphysicalvolumesize n" but it says /dev/sda2: cannot resize to n extents as later ones are allocated." no matter how BIG is n. What I am doing wrong ?

GazL · 03-18-2009, 06:32 AM

If you look at the output of "pvdisplay --maps" on your PVdevice you'll probably find that one or more of the LVs are using PEs above the cutoff point you're trying to shrink it to. Mine is like this too:

Code:

root@jupiter:~# pvdisplay --maps /dev/mapper/pvhda2
  --- Physical volume ---
  PV Name               /dev/mapper/pvhda2
  VG Name               sysvg
  PV Size               18.91 GB / not usable 2.42 MB
  Allocatable           yes 
  PE Size (KByte)       4096
  Total PE              4841
  Free PE               1769
  Allocated PE          3072
  PV UUID               oSsu4H-HNk8-TRt0-yXJU-AoDP-82bF-ajtHrQ
   
  --- Physical Segments ---
 Physical extent 0 to 1279:
    FREE
  Physical extent 1280 to 1407:
    Logical volume      /dev/sysvg/lvswap
    Logical extents     0 to 127
  Physical extent 1408 to 1663:
    FREE
  Physical extent 1664 to 1791:
    Logical volume      /dev/sysvg/lvtmp
    Logical extents     0 to 127
  Physical extent 1792 to 2303:
    Logical volume      /dev/sysvg/lvhome
    Logical extents     0 to 511
  Physical extent 2304 to 4351:
    Logical volume      /dev/sysvg/lvroot
    Logical extents     0 to 2047
  Physical extent 4352 to 4607:
    Logical volume      /dev/sysvg/lvvar
    Logical extents     0 to 255
Physical extent 4608 to 4840:
    FREE
   
root@jupiter:~#

As you can see, even though I've only allocated 3072 PE's, they're spread all over the disk.

I've never tried this, but looking at the pvmove man page it looks like its possible to explicitly specify the PE numbers when you move, so assuming pvmove will allow you to move using the same PV for source and destination it may be able to move the PEs down below the point you want to resize too.

DISCLAIMER: THIS IS JUST THEORY, I'VE NEVER TRIED THIS, SO MAKE SURE YOU HAVE A GOOD BACKUP IF YOU DECIDE TO HAVE A GO. YOU COULD LOSE IT ALL

Anyway, hopefully that has given you some ideas.

Good luck.

marozsas · 03-18-2009, 08:32 PM

Thank you GazL,

You right !
My LVM setup is such it is using PE all around the PV, so I couldn't resize it.
I didn't try to move PE because they are so many and as you said, there are a big risk to lose my data.
If I have to backup my data first, I decide it is better (in fact, is is more simple, easy) to remove the entire partition and re-create it using a reasonable size and initialize LVM on this partition again and restore the data from the backup.

So, this is what I did do. And it worked !

LVM is good to expand, but not so good to shrink, isn't ?

I can't wait for ZFS or other more advanced FS to come.

best regards,

GazL · 03-19-2009, 05:27 AM

Quote:

Originally Posted by marozsas

If I have to backup my data first, I decide it is better (in fact, is is more simple, easy) to remove the entire partition and re-create it using a reasonable size and initialize LVM on this partition again and restore the data from the backup.

So, this is what I did do. And it worked !

That seems like a sensible approach. I'd have probably done something similar too.

Quote:

Originally Posted by marozsas

LVM is good to expand, but not so good to shrink, isn't ?

I can't wait for ZFS or other more advanced FS to come.

LVM is wonderful, I couldn't live without it, but it's purpose is to allow you to dynamically manage filesystems at the LV level. The PVs are really meant to be mapped onto physical disks which aren't expected to be subject to resizing. On a single OS system, you'd never have a reason to resize a PV. It's only when you start using it on a dual boot PC with its disk partitions that changing a PVs size ever becomes a consideration.

ZFS sounds quite nice, but from what I've read it's not perfect and has its own restrictions and management issues. It also appears that it requires a serious amount of free system memory to stop it running out of buffers and panicking the kernel. According to the FreeBSD guys who have played with it, it's not really recommended for a 32bit system. In contrast, my LVM setup runs nicely on this antique Pentium3 800Mhz with 128MB ram, and that's with the PVs all on LUKS encrypted partitions too.

Anyway, I'm glad you've got it all sorted now.

Have fun...