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Full Discussion: Issue about PP size
Operating Systems AIX Issue about PP size Post 302757065 by MichaelFelt on Thursday 17th of January 2013 04:21:04 AM
Old 01-17-2013
The differences in PP - Physical Partition - sizes is caused by a number of defaults or constants re: the meta data for managing the volumes (aka disks) in a volume group.

The default size for a PP is actually 4M - dating back to when disks were smaller than 4G - because the default number of PP per volume has remained the same - 1016.

When disks were 4G and smaller the VGDA (Volume Group Descriptor Area) was 32M Byte large (or 8 x 4M, so a volume was 32M for VGDA (i.e. 8PP) with upto 1016 (1024 - 8) PP for volume group data.

The VGDA is an important structure - there are three kinds or sizes: normal, big and scaled. Normal is the original and it has space for 32 volumes of 1016 PP each, the big for 128 volumes of 1016 PP each, and the scaled originally began as 1024 disks of 1016 PP each, but now it is meant to scale/grow the max number of PP as the volume grows. But still the original factor of PP per disk is 1016.

Early on (with 4.5 and 9.1 G disks) AIX made changes to affect the number of PP per disk, e.g., so the PP size could be held constant even though disk sizes changed - however, the total number of PP per volume group (a VGDA constant) was fixed. The effect is that changing the number of PP per disk (the -t option of mkvg) means fewer disks can be included in a volume group (normal, and big are fixed at 32 and 128 by defaults).

So, basically, to keep a constant PP size over multiple disk sizes you need to divide the disk size by that PP size to see how many PP that will need, divide that number by 1016, round that number up to the next whole number (i.e., 2033 will need a factor of 3, or 3048 PP (-t 2 if I recall correctly) per volume while 2031 or 2032 will work with a factor of -t 1

Code:
-t 0 - default 1016
-t 1 - 2032 
-t 2 - 3048 
-t 3 - 4064
-t 4 - 5080
etc

Note: adding a disk larger than the original will mean it will not be fully utilized, however, you may change the scaling to a higher number dynamically (say you have been using 75G as a default, and want to add 150G disk/LUN. With no change, only 75G of the disk would be used. Change the scaling: the max number of disks declines, but the 150G disk can be fully utilized.

IMPOORTANT: you cannot change the PP size once a volume group is created. To change the PP size you must backup/restore to a new volume group.

Hope this helps!
 

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LEARN ABOUT REDHAT

vgextend

VGEXTEND(8)						      System Manager's Manual						       VGEXTEND(8)

NAME

       vgextend - add physical volumes to a volume group

SYNOPSIS

       vgextend [-A|--autobackup{y|n}] [-d|--debug] [-h|--help] [-v|--verbose] VolumeGroupName PhysicalVolumePath [PhysicalVolumePath...]

DESCRIPTION

       vgextend allows you to add one or more initialized physical volumes ( see pvcreate(8) ) to an existing volume group to extend it in size.

   OPTIONS
       -A, --autobackup y/n
	      Controls automatic backup of VG metadata after the change ( see vgcfgbackup(8) ). Default is yes.

       -d, --debug
	      Enables additional debugging output (if compiled with DEBUG).

       -h, --help
	      Print a usage message on standard output and exit successfully.

       -v, --verbose
	      Gives verbose runtime information about lvextend's activities.

Examples
       "vgextend  vg00	/dev/sda4  /dev/sdn1"  tries  to  extend  the  existing volume group "vg00" by the new physical volumes (see pvcreate(8) )
       "/dev/sdn1" and /dev/sda4".

DIAGNOSTICS

       vgextend returns an exit code of 0 for success and > 0 for error:

       1  no volume group name on command line
       2  volume group name not given as first argument
       3  no physical volume(s) on command line
       4  invalid volume group name
       5  error checking existence of volume group
       6  inactive volume group
       7  error reading VGDA
       8  volume group is set to not extendable
       9  maximum physical volume number of volume group exceeded
       10 error reading all physical volumes
       11 error setting up VGDA
       12 error extending VGDA in kernel
       13 error writing VGDA to physical volume(s)

       95 driver/module not in kernel
       96 invalid I/O protocol version
       97 error locking logical volume manager
       98 invalid lvmtab (run vgscan(8))
       99 invalid command line

ENVIRONMENT VARIABLES

       LVM_AUTOBACKUP
	      If this variable is set to "no" then the automatic backup of VG metadata is turned off.

       LVM_VG_MAX_BACKUPS
	      This variable determines the backup history depth of kept VGDA copy files in /etc/lvmconf. It  can  be  set  to  a  positive  number
	      between 0 and 999.  The higher this number is, the more changes you can restore using vgcfgrestore(8).

See also
       lvm(8), vgcreate(8), vgreduce(8), pvcreate(8)

AUTHOR

       Heinz Mauelshagen <Linux-LVM@Sistina.com>

Heinz Mauelshagen						     LVM TOOLS							       VGEXTEND(8)
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