LVM - The Size of the Volume Group Descriptor Area

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In some instances, the user will experience a problem adding a new disk to an existing volume group or creating a new volume group. The warning message provided by LVM (Logincal Volume Manager) will be:

   Not enough descriptor space left in this volume group. 
   Either try adding a smaller PV or use another volume group. 

On every disk in a volume group, there exists an area called the Volume Group Descriptor Area (VGDA). This space is what allows the user to take a volume group to another AIX system and importvg the volume group into another AIX system. The VGDA contains the names of disks that make up the volume group, their physical sizes, partition mapping, logical volumes that exist in the volume group, and other pertinent LVM management information.

When the user creates a volume group, the mkvg command defaults to allowing a maximum of 32 disks in the new volume group. However, as bigger disks have become more prevalent, this 32-disk limit is usually not achieved because the space in the VGDA is used up faster, as it accounts for the capacity on the bigger disks. This maximum VGDA space is a fixed size which is part of the LVM design. Large disks require more management mapping space in the VGDA, which causes the number and size of available disks to be added to the existing volume group to shrink. When a disk is added to a volume group, not only does the new disk get a copy of the updated VGDA, but all existing drives in the volume group must be able to accept the new, updated VGDA.

The exception to this description of the maximum VGDA is rootvg. In order to provide AIX users more free space, when rootvg is created mkvg does not use the maximum limit of 32 disks that are allowed into a volume group. Instead, the number of disks picked in the install menu of AIX is used as the reference number by mkvg -d during the creation of rootvg. This limit does not mean the user cannot add more disks to rootvg in the post-install phase. The amount of free space left in a VGDA, and thus the number and size of the disks added to a volume group, depends on the size and number of disks already defined for a volume group. However, this smaller size during rootvg creation implies that the user will be able to add fewer disks to rootvg than compared to a non-rootvg volume group.

Following is the formula used to determine how many physical partitions can be potentially mapped in rootvg, based on the number of disks selected during the mksysb install or during a fresh installation:

    AIX 3.2:  Total PPs = Number of install disks * 1016 PPs/disk 
    AIX 4.1:  Total PPs = 32 disks * 1016 PPs/disk 
    AIX 4.2:  Total PPs = 32 disks * 1016 PPs/disk 
    AIX 4.3:  Total PPs = 32 disks * 1016 PPs/disk 

At AIX 4.1 or 4.2, if the amount of RAM is 8MB or less, the formula would be as follows:

    Total PPs = (Number of disks + 3) * 1016 

At AIX 4.1 APAR IX69937 is needed to achieve the total amount of PPs given above. If IX69937 is NOT installed prior to creating the mksysb, and the amount of system RAM is greater then 8MB, then the following formula will apply:

    Total PPs = (Number of disks + 7) * 1016 PPs/disk 

If more VGDA space is needed in the rootvg, the user will need to reinstall the operating system from installation media, or a bootable backup tape onto the additional drives will be needed.

If the system has been migrated to AIX Version 4.3.2, the situation can also be alleviated by converting rootvg to a BIGVG enabled volume group with the chvg -B rootvg command. Please refer to the documentation for chvg regarding requirements and limitations.

NOTE: It is always strongly recommended that users do not place user data onto rootvg disks. This separation provides an extra degree of system integrity.