Portability of volume groups
The one great attribute of LVM is the ability of the user to take a disk or sets of disks that make up a volume group and take it to another RS/6000 system and introduce the information created on another machine onto the second machine. This ability is provided through the Volume Group Descriptor Area (VGDA) and the logical volume control block (lvcb). The design of LVM also allows for accidental duplication of volume group and logical volume names. If on the new machine, the volume group or logical volume names being imported already exist, then LVM will generate a distinct volume group or logical volume name.
Mix and Match disks
As mentioned before, LVM allows the user to attach disks to volume group, regardless of what type of physical device it true is or what type of device driver is used to operate the device. Thus, RAID systems, serial dasd drives, and the plain SCSI drives can all make up one volume group that may reside across several adapters. The physical location of each drive and the true nature of the drive doesn’t matter to LVM as long as the disk device drivers follow a certain format required by LVM in order to create logical volumes on those drives.
Volume Group Descriptor Area
The VGDA is an area at the front of each disk which contains information about the volume group, the logical volumes that reside on the volume group and disks that make up the volume group. For each disk in a volume group, there exists a VGDA concerning that volume group. This VGDA area is also used in quorum voting. The VGDA contains information about what other disks make up the volume group. This information is what allows the user to just specify one of the disks in the volume group when they are using the “importvg” command to import a volume group into an AIX system. The importvg will go to that disk, read the VGDA and find out what other disks (by PVID) make up the volume group and automatically import those disks into the system (and its) ODM as well. The information about neighboring disks can sometimes be useful in data recovery. For the logical volumes that exist on that disk, the VGDA gives information about that logical volume so anytime some change is done to the status of the logical volume (creation, extension, or deletion), then the VGDA on that disk and the others in the volume group must be updated.
Volume Group Status Area
The Volume Group Status Area (VGSA) is comprised of 127 bytes, where each bit in the bytes represents up to 1016 Physical Partitions that reside on each disk. The bits of the VGSA are used as a quick bit-mask to determine which Physical Partitions, if any, have become stale. This is only important in the case of mirroring where there exists more than one copy of the Physical Partition. Stale partitions are flagged by the VGSA. Unlike the VGDA, the VGSA’s are specific only to the drives which they exist. They do not contain information about the status of partitions on other drives in the same volume group. The VGSA is also the bit masked used to determine which physical partitions must undergo data resyncing when mirror copy resolution is performed.
Quorum
Quorum is a sort of “sanity” check that LVM uses to resolve possible data confliction and prevent data corruption. Quorum is a method by which 51% or more quorum votes must be available to a volume group before LVM actions can continue. Quorum is issued to a disk in a volume group according to how the disk was created within the volume group. When a volume group consists of one disk, there are two VGDA’s on that disk. Thus, this single disk volume group has a quorum vote of 2. When another disk is added to the volume group with an “extendvg”, then this new disk gets one VGDA, but the original, first disk still retains the two VGDA’s. When the volume group has been extended to three disks, the third disk gets the spare VGDA sitting on the first disk and then each disk has a quorum vote of 1. Every disk after the third disk is automatically given one VGDA, and thus one vote.
VGID
Just as the PVID is a soft serial number for a disk, the VGID is the soft serial number for the volume group. It is this serial number, not the volume group’s ascii name, which all low level LVM commands reference. Additionally, it is the basis for the LVIDs created on that VGID.
A Graphical View of VGDA Expansion
This shows how the VGDA grows from one disk to another and how information is carried in the VGDA which crossreferences information among the disks in a volume group. The exact VGDA structure is not defined in this section because it is quite complex and contains IBM confidential information.