Veritas Admin Reference – Important Definitions of Veritas Volume Manager

vxvm-how-to Below information will give you quick summary of VxVM terminology and the definitions. Understanding these definitions will help you to quickly learn the VxVM concepts.

Boot disk group  :

If the root disk is under VxVM control, this disk group contains the root disk and the volumes on it that are used to boot the system. By default, the name of the boot disk group is aliased to the reserved disk group name, bootdg, when the root disk is put under VxVM control.

Concatenated plex  :

A plex whose subdisks are associated at specific offsets within the address range of the plex, and extend into the plex address range for the length of the subdisk. This layout allows regions of one or more disks to create a plex, rather than a single big region.
data change map :
The data change map (DCM) is a bit-map which represents regions in the volume. When a write to a region occurs, the corresponding bit in the DCM is turned on. It is used by the Volume Replicator (VVR) for both log overflow protection and automatic secondary synchronization.

Data volume :

A volume being used as a child of a replicated volume group (RVG). For a primary RVG, a data volume contains the primary copy of that volume data. For a secondary RVG, a data volume contains a copy of the corresponding remote primary data volume. Secondary data volumes are only writable with updates from the primary. The secondary RVG must contain a data volume corresponding to each primary data volume.

Default disk group  :

Each system may have one special disk group, aliased to the reserved name, defaultdg, which is the default disk group for most utilities.

Disk :

Disks exist as two entities. One is the physical disk on which all data is ultimately stored and which exhibits all the behaviors of the underlying technology. The other is the Veritas Volume Manager presentation of disks which, while mapping one-to-one with the physical disks, are just presentations of units from which allocations of storage are made. As an example, a physical disk presents the image of a device with a definable geometry with a definable number of cylinders, heads, and so forth, whereas a Veritas Volume Manager disk (VM disk) is simply a unit of allocation with a name and a size.

Disk access record :

A configuration record that defines a pathway to a disk. The disk access records most commonly name a particular controller number, target ID, and logical unit number. The list of all disk access records stored in a system is used to find all disks attached to the system. Disk access records do not identify particular physical disks.
Disk access records are identified by their disk access names (also known as DA names).
Through the use of disk IDs, VxVM allows disks to be moved between controllers, or to different locations on a controller. When a disk is moved, a different disk access record is used when accessing the disk, although the disk media record continues to track the actual physical disk.
On some systems, VxVM builds a list of disk access records automatically, based on the list of all devices attached to the system. On these systems, it is not necessary to define disk access records explicitly. On other systems, disk access records must be defined explicitly with the vxdisk define operation. Specialty disks (such as RAM disks or floppy disks) are likely to require explicit vxdisk define operations on all systems.

Disk group :

A group of disks that share a common configuration. A configuration consists of a set of records describing objects (including disks, volumes, plexes, and subdisks) that are associated with one particular disk group. Each disk group has an administrator-assigned name that can be used by the administrator to reference that disk group. Each disk group has an internally defined unique disk group ID, which is used to differentiate two disk groups with the same administrator-assigned name.
Disk groups provide a method to partition the configuration database, so that the database size is not too large and so that database modifications do not affect too many drives. They also allow VxVM to operate with groups of physical disk media that can be moved between systems.
Disks and disk groups have a circular relationship: disk groups are formed from disks, and disk group configurations are stored on disks. All disks in a disk group are stamped with a disk group ID, which is a unique identifier for naming disk groups. Some or all disks in a disk group also store copies of the configuration database of the disk group.

Disk group configuration :

A disk group configuration is a small database that contains all volume, plex, subdisk, and disk media records. These configurations are replicated onto some or all disks in the disk group, usually with one copy on each disk. Because these databases are stored within disk groups, record associations cannot span disk groups. Thus, a subdisk defined on a disk in one disk group cannot be associated with a volume in another disk group.

Disk header :

A block stored in a private region of a disk and that defines several properties of the disk. The disk header defines the size of the private region, the location and size of the public region, the unique disk ID for the disk, the disk group ID and disk group name (if the disk is currently associated with a disk group), and the host ID for a host that has exclusive use of the disk.
disk ID A 64-byte universally unique identifier that is assigned to a physical disk when its private region is initialized with the vxdisk init operation. The disk ID is stored in the disk media record so that the physical disk can be related to the disk media record at system startup.

Disk media record :

A reference to a physical disk or a disk partition This record can be thought of as a physical disk identifier for the disk or partition Disk media records are configuration records that provide a name with up to 31 characters (known as the disk media name or DM name), which you can use to reference a particular disk, independent of its location on the system’s various disk controllers. Disk media records reference particular physical disks through a disk ID, which is a unique identifier that is assigned to a disk when it is initialized for use with VxVM.
Operations are provided to set or remove the disk ID stored in a disk media record. Such operations have the effect of removing or replacing disks, with any associated subdisks being removed or replaced along with the disk.

Host ID :

A name, usually assigned by the administrator, that identifies a particular host. Host IDs are used to assign ownership to particular physical disks. When a disk is part of a disk group that is in active use by a particular host, the disk is stamped with that host’s host ID. If another system attempts to access the disk, it detects that the disk has a non-matching host ID and disallows access until the first system discontinues use of the disk. To allow for system failures that do not clear the host ID, the vxdisk clearimport operation can be used to clear the host ID stored on a disk.
If a disk is a member of a disk group and has a host ID that matches a particular host, then that host imports the disk group as part of system startup.
kernel log A log kept in the private region on the disk and that is written by the Veritas Volume Manager kernel. The log contains records describing the state of volumes in the disk group. This log provides a mechanism for the kernel to persistently register state changes so that vxconfigd can be guaranteed to detect the state changes even in the event of a system failure.

Layered volume :

A virtual volume that is used and managed directly by VxVM, not by a user. Note that currently a layered volume provides storage for only one upper-level volume.
Layered volumes allow VxVM to implement the following features:
Striped mirrors and concatenated mirrors are new types of volume layouts that are less likely to fail and that provide faster recovery times because there is less data to recover (one column or part of a column versus the whole volume).
Online relayout lets you change the volume layout while the volume is online. See the vxassist  and vxrelayout manual pages for more information.
RAID-5 subdisk moves and RAID-5 snapshots .

Plex :

A copy of a volume’s logical data address space, also called a mirror. A volume can have up to 32 plexes associated with it. Each plex is, at least conceptually, a copy of the volume that is maintained consistently in the presence of volume I/O and reconfigurations. Plexes represent the primary means of configuring storage for a volume. Plexes can have a striped, concatenated, or RAID-5 organization (layout).

Plex Consistency :

If the plexes of a volume contain different data, then the plexes are said to be inconsistent. This is only a problem if VxVM is unaware of the inconsistencies, as the volume can return differing results for consecutive reads. Plex inconsistency is a serious compromise of data integrity. This inconsistency can be caused by write operations that start around the time of a system failure, if parts of the write complete on one plex but not the other. Plexes can also be inconsistent after creation of a mirrored volume, if the plexes are not first synchronized to contain the same data. An important part of Veritas Volume Manager operation is ensuring that consistent data is returned to any application that reads a volume. This may require that plex consistency of a volume be ‘‘recovered’’ by copying data between plexes so that they have the same contents. Alternatively, the volume can be put into a state such that reads from one plex are automatically written back to the other plexes, thus making the data consistent for that volume offset.

Private region :

Disks used by VxVM contain two special regions: a private region and a public region. Usually, each region is formed from a complete partition of the disk; however, the private and public regions can be allocated from the same partition.
The private region of a disk contains various on-disk structures that are used by VxVM for various internal purposes. Each private region begins with a disk header which identifies the disk and its disk group. Private regions can also contain copies of a disk group’s configuration, and copies of the disk group’s kernel log.
public region : The public region of a disk is the space reserved for allocating subdisks. Subdisks are defined with offsets that are relative to the beginning of the public region of a particular disk. Only one contiguous region of disk can form the public region for a particular disk.

RLINK(remote link) :

A representation of a communications link to a RVG hierarchy at a remote replication site, which contains information about the link. An RLINK is a primary RLINK if its parent RVG is the primary RVG containing writable primary data volumes. Alternatively, a RLINK is a secondary RLINK if its parent RVG contains read-only data volumes (one for each data volume that the primary RVG contains). The vxrlink command is used to replicate a volume or volumes to any number of remote sites. The primary RVG has one RLINK record associated with it for each secondary site. A secondary RVG has only one RLINK record associated with it, which represents and contains information about the connection with the corresponding primary RLINK.

RVG (replicated volume group):

A virtual device that hierarchically contains one or more data volume records, a log volume record called a Storage Replicator Log (SRL), and one (or more for primary RVGs) RLINK records. It must be associated as a parent of the data volume records in order for those data volumes to be replicated to other sites which may be located across a LAN or WAN. In order to actively replicate a data volume, an RVG must have at least one data volume child (the volume to be replicated), exactly one SRL volume child (a volume to SRL data volume writes before they are transmitted to RLINKs), and at least one RLINK child (which represents a connection to an RVG hierarchy at a remote replication site). An RVG can be either a primary or a secondary, depending on whether its data volumes are considered to contain the primary copies of data or whether the data volumes are considered to be read-only copies of the data. Secondary RVGs contain one SRL volume, one RLINK, and the same number of data volumes as the primary RVG contains.

SRL volume(storage replicator log volume):

A volume being used as a child of an RVG. The SRL volume contains temporary copies of data intended to be written from (primary) or to (secondary) sibling data volumes. The SRL volume also contains meta data, such as connection information, about the RVG and RLINK with which the SRL volume is associated. The primary SRL volume is used to log SRL data while either in asynchronous mode, or during network outages while in synchronous mode. A secondary RVG must also have an SRL volume associated with it.

Striped plex :

A plex that distributes data evenly across each of its associated subdisks. A plex has a characteristic number of stripe columns (consisting of associated subdisks) and a characteristic stripe unit size. The stripe unit size defines how data with a particular address is allocated to one of the associated subdisks. Given a stripe unit size of 128 blocks and two stripe columns, the first group of 128 blocks is allocated to the first subdisk, the second group of 128 blocks is allocated to the second subdisk, the third group to the first subdisk, and so on.

Subdisk :


A region of storage allocated on a disk for use with a volume. Subdisks are associated to volumes through plexes. One or more subdisks are laid out to form plexes based on the plex layout (striped, concatenated, or RAID-5). Subdisks are defined relative to disk media records.
volboot file : The volboot file is a special file (usually stored in /etc/vx/volboot) that is used to define a system’s host ID and a system-wide default disk group. If the root disk is under VxVM control, the volboot file is also used to define the name of the boot disk group for the system.

Volume :

A virtual disk device that looks to applications and file systems like a regular disk partition device. Volumes present block and raw device interfaces that are compatible in their use with disk partition devices. However, a volume is a virtual device that can be mirrored, spanned across disk drives, moved to use different storage, and striped using administrative commands. The configuration of a volume can be changed, using Veritas Volume Manager utilities, without causing disruption to applications or file systems that are using the volume.


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1 Response

  1. September 18, 2015

    […] Read – Important Definitions of VxVM […]

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