This document discusses the volume group descriptor area (VGDA), its default
size, and the "not enough descriptor space" error. It explains why the VGDA
may run out of space before the limit of 32 disks per volume group is met. This
document applies to AIX 3.2, 4.1, 4.2, and 4.3.
http://www.rs6000.ibm.com/resource/aix_resource/Pubs/index.html
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 (Logical Volume Manager) will be:
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:
At AIX 4.1 or 4.2, if the amount of RAM is 8MB or less, the formula would be
as follows:
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:
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.
[ Doc Ref: 90605226614702 Publish Date: Oct. 09, 2000 4FAX Ref: 3690 ]
Related Documentation
The product documentation library is also available:
"Not enough descriptor space"
Not enough descriptor space left in this volume group.
Either try adding a smaller PV or use another volume group.
VGDA Overview
Default VGDA Size
rootvg
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
Total PPs = (Number of disks + 3) * 1016
Total PPs = (Number of disks + 7) * 1016 PPs/disk