LVM Limitation of Physical Partitions per Physical Volume

Answer

This document discusses the limit of 1016 physical partitions per disk and how this limit affects the addition of disks that require the creation of more than 1016 physical partitions when using the mkvg or extendvg commands.

NOTE: This discussion does not cover the concept of Scaleable Volume Groups, introduced in AIX 5.3. A comprehensive discussion about this type of volume group can be found in the IBM RedBook, AIX 5L Differences Guide: Version 5.3 Edition.

This document applies to AIX versions 4 and 5.

Overview

In the design of Logical Volume Manager (LVM), each logical partition maps to one physical partition, and each physical partition maps to one or more physical partitions. Each physical partition maps to a number of disk sectors (only when mirroring, one LP maps to two or three physical partitions). The design of LVM limits the number of physical partitions that LVM can track per disk to 1016. In most cases, not all the possible 1016 tracking partitions are used by a disk. The default size of each physical partition during a mkvg command is 4MB for AIX V4, which implies that individual disks up to 4GB can be correctly tracked in a volume group.

NOTE: In AIX V5, the mkvg command has been enhanced to automatically determine the PP size when creating a new volume group.

Adding disks that require more than 1016 PPs/PV

When trying to add a physical volume to a volume group, and it requires the creation of more than 1016 physical partitions per physical volume (PPs/PV), the disk addition will fail with a warning message similar to:

     0516-1162 extendvg: Warning, The Physical Partition Size of 8 
     requires the creation of 2172 partitions for hdisk13.  The limitation 
     for volume group vgname is 1016 physical partitions per physical 
     volume.  Use chvg command with -t option to attempt to change the 
     maximum Physical Partitions per Physical volume for this volume group. 

     0516-792 extendvg: Unable to extend volume group.

In the above example, vgname uses a PP size of 8 MB and hdisk13 is 17378MB in size.

There are two instances where this limitation will be enforced.

The first case applies only to AIX V4 when the user tries to use mkvg to create a volume group in which the number of physical partitions on one of the disks in the volume group would exceed 1016. In this case, the user must pick from the available physical partition size, which ranges as follows:

   
     1, 2, (4), 8, 16, 32, 64, 128, or 256 MB

The user must specify the pp size using the -s option with mkvg.

The second case applies to AIX versions 4 and 5 when the user attempts to use extendvg to add a disk that violates the 1016 limitation with a pre-existing volume group. In this case, the user has three options to choose from:

1. Backup the volume group, remove and recreate it with a larger physical partition size (which will allow the new disk to work with the 1016 limitation).
2. The user can create a standalone volume group (consisting of a larger physical partition size) for the new disk.
3. Increase the factor size for the volume group using the -t flag with the chvg command. For more details, see the section in this document on How to increase the factor size.

Frequently asked questions

• Can I move this volume group between systems and versions of AIX?

  Yes. The enforcement of the 1016 limit occurs only during mkvg and extendvg.

How to increase the factor size

AIX V4 and later versions relaxed the limitation of 1016 physical partitions per physical volume, by introducing the concept of a volume group factor. When creating a volume group, you can specify your own PP/PV limitation, in multiples of 1016 (that is, 1016, 2032, 3048, etc). This is done via the -t flag on mkvg.

For example, the following command will create a volume group that will allow up to 2032 PPs/PV with the default PP size of 4MB at AIX V4. In AIX V5, mkvg automatically determines the PP size if one is not specified.

     mkvg -y VGname -t 2 hdisk1

The disadvantage is that increasing the number of PPs allowed per disk will limit the number of disks allowed in the volume group. If the 1016 PP/PV limit is maintained, you can add up to 32 physical volumes in a standard volume group or up to 128, if it is a "BIG" volume group.

The chvg command also has a -t flag to allow you to change the max PPs/PV limit on an existing volume group, so that a larger drive could be added at that point.

Example:

     chvg -t 2 VGname

Only volume groups that have been created with a factor size other than 1, or that have been to a new factor size, can exceed the 1016 PP/PV limit.

Any volume group created outside the default factor size or changed to a different factor CANNOT be used on systems prior to AIX 4.3.1, even if the factor size is later changed back to 1. An example of the error received when accessing the volume group will look like the following:

     0516-002 lqueryvg: The volume group version is incompatible 
     with this level of the operating system and cannot be activated.

NOTE: These options are only available from the command line, not from SMIT.