How Does Volume Manager Work?

With Volume Manager, you enable virtual data storage by bringing a disk under Volume Manager control. To bring a disk under Volume Manager control means that Volume Manager creates virtual objects and establishes logical connections between those objects and the underlying physical objects, or disks.

When a disk is brought under Volume Manager control:

1. Volume Manager removes all of the partition table entries from the VTOC, except for partition table entry 2 (backup slice). Partition table entry 2 contains the entire disk, including the VTOC, and is used to determine the size of the disk.

Note: The boot disk is a special case and is discussed in a later lesson.

2. Volume Manager then rewrites the VTOC and creates two partitions on the physical disk. One partition contains the private region, and the other contains the public region.
   • Private region: The private region stores information, such as disk headers, configuration copies, and kernel logs, that Volume Manager uses to manage virtual objects. The private region represents a small management overhead.

The minimum size for the private region is 1024 sectors (512K) for disks with active configuration databases, but VxVM uses 2048 sectors (1024K) by default. This default value is rounded up to the next cylinder boundary.

The maximum size for the private region is 524288 blocks (512K sectors).

• Public region: The public region consists of the remainder of the space on the disk. The public region represents the available space that Volume Manager can use to assign to volumes and is where an application stores data. Volume Manager never overwrites this area unless specifically instructed to do so.

By convention, the public region of a Volume Manager-controlled disk is referred to as a Volume Manager disk, or VxVM disk. The true definition of a VxVM disk is a Volume Manager-controlled disk.

3. Partition Tags: VxVM sets the partition tags, the numeric values that describe the file system mounted on a partition, for the public and private regions:
   • Tag 14 is always used for the public region of the disk.
   • Tag 15 is always used for the private region of the disk.

If the disk has no partitions that are being placed under Volume Manager control, then Volume Manager creates the private region first, and the public region second, on the disk.

4. Volume Manager updates the VTOC with information about the removal of the existing partitions and the addition of the new partitions during the initialization process.

Source: VERITAS Volume Manager Fundamentals I

ERITAS Volume Manager Fundamentals I