Volume Manager Storage Objects

Disk Groups:
A disk group is a collection of VxVM disks that share a common configuration. Disk groups are configured by the system administrator and represent management and configuration boundaries. VM objects cannot span disk groups.

Disk groups ease the use of devices in a high availability environment, because a disk group and its components can be moved as a unit from one host machine to another. Disk drives can be shared by two or more hosts, but can be accessed by only one host at a time.

Volume Manager Disks:
A Volume Manager (VxVM) disk represents the public region of a physical disk that is under Volume Manager control. Each VxVM disk corresponds to one physical disk. Each VxVM disk has a unique virtual disk name called a disk media name. VM uses the disk media name when assigning space to volumes. A VxVM disk is given a disk media name when it is added to a disk group.

Subdisks:
A subdisk is a set of contiguous disk blocks that represent a specific portion of a VxVM disk, which is mapped to a specific region of a physical disk. A subdisk is a subsection of a disk’s public region. A subdisk is the smallest unit of storage in VM.

Plexes:
A plex is a structured or ordered collection of subdisks that represent one copy of the data in a volume. A plex consists of one or more subdisks located on one or more physical disks.

Plex types:
Complete plex – A complete plex holds a complete copy of a volume and therefore maps the entire address space of the volume.

Sparse plex – A sparse plex is a plex that has a length that is less than the length of the volume or that map to only part of the address space of a volume.

Log plex – A log plex is a plex that is dedicated to logging. A log plex is used to speed up data consistency checks and repairs after system failure. RAID-5 and mirrored volumes typically use a log plex.

A volume must have at least one complete plex that has a complete copy of the data in the volume with at least one associated subdisk. Other plexes in the volume can be complete, sparse, or log plexes. A volume can have up to 32 plexes; however, you should never use more than 31 plexes in a single volume. Volume manager requires one plex for automatic or temporary online operations.

Volumes:
A volume is a virtual storage device that is used by applications in a manner similar to a physical disk. A VxVM volume can be as large as the total of available, unreserved free physical disk space in the disk group. A volume is comprised of one or more plexes.