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Active/Active disk arrays

This type of multipathed disk array allows you to access a disk in the disk array through all the paths to the disk simultaneously, without any performance degradation.

Active/Passive disk arrays

This type of multipathed disk array allows one path to a disk to be designated as primary and used to access the disk at any time. Using a path other than the designated active path results in severe performance degradation in some disk arrays.


The process of establishing a relationship between VxVM objects; for example, a subdisk that has been created and defined as having a starting point within a plex is referred to as being associated with that plex.

associated plex

A plex associated with a volume.

associated subdisk

A subdisk associated with a plex.

atomic operation

An operation that either succeeds completely or fails and leaves everything as it was before the operation was started. If the operation succeeds, all aspects of the operation take effect at once and the intermediate states of change are invisible. If any aspect of the operation fails, then the operation aborts without leaving partial changes.

In a cluster, an atomic operation takes place either on all nodes or not at all.


A state in which a VxVM object is both associated with another object and enabled for use.


The minimum unit of data transfer to or from a disk or array.

boot disk

A disk that is used for the purpose of booting a system.

boot disk group

A private disk group that contains the disks from which the system may be booted.


A reserved disk group name that is an alias for the name of the boot disk group.

clean node shutdown

The ability of a node to leave a cluster gracefully when all access to shared volumes has ceased.


A set of hosts (each termed a node) that share a set of disks.

cluster manager

An externally-provided daemon that runs on each node in a cluster. The cluster managers on each node communicate with each other and inform VxVM of changes in cluster membership.

cluster-shareable disk group

A disk group in which access to the disks is shared by multiple hosts (also referred to as a shared disk group).


A set of one or more subdisks within a striped plex. Striping is achieved by allocating data alternately and evenly across the columns within a plex.


A layout style characterized by subdisks that are arranged sequentially and contiguously.

configuration copy

A single copy of a configuration database.

configuration database

A set of records containing detailed information on existing VxVM objects (such as disk and volume attributes).

data change object (DCO)

A VxVM object that is used to manage information about the FastResync maps in the DCO volume. Both a DCO object and a DCO volume must be associated with a volume to implement Persistent FastResync on that volume.

data stripe

This represents the usable data portion of a stripe and is equal to the stripe minus the parity region.

DCO volume

A special volume that is used to hold Persistent FastResync change maps, and dirty region logs (see dirty region logging).


A state in which a VxVM object is associated with another object, but not enabled for use.

device name

The device name or address used to access a physical disk, such as hdisk3, which indicates the whole of disk 3.

In a SAN environment, it is more convenient to use enclosure-based naming, which forms the device name by concatenating the name of the enclosure (such as enc0) with the disk's number within the enclosure, separated by an underscore (for example, enc0_2). The term disk access name can also be used to refer to a device name.

dirty region logging

The method by which the VxVM monitors and logs modifications to a plex as a bitmap of changed regions. For a volumes with a new-style DCO volume, the dirty region log (DRL) is maintained in the DCO volume. Otherwise, the DRL is allocated to an associated subdisk called a log subdisk.

disabled path

A path to a disk that is not available for I/O. A path can be disabled due to real hardware failures or if the user has used the vxdmpadm disable command on that controller.


A collection of read/write data blocks that are indexed and can be accessed fairly quickly. Each disk has a universally unique identifier.

disk access name

An alternative term for a device name.

disk access records

Configuration records used to specify the access path to particular disks. Each disk access record contains a name, a type, and possibly some type-specific information, which is used by VxVM in deciding how to access and manipulate the disk that is defined by the disk access record.

disk array

A collection of disks logically arranged into an object. Arrays tend to provide benefits such as redundancy or improved performance.

disk array serial number

This is the serial number of the disk array. It is usually printed on the disk array cabinet or can be obtained by issuing a vendor- specific SCSI command to the disks on the disk array. This number is used by the DMP subsystem to uniquely identify a disk array.

disk controller

In the multipathing subsystem of VxVM, the controller (host bus adapter or HBA) or disk array connected to the host, which the operating system represents as the parent node of a disk.

disk enclosure

An intelligent disk array that usually has a backplane with a built-in Fibre Channel loop, and which permits hot-swapping of disks.

disk group

A collection of disks that share a common configuration. A disk group configuration is a set of records containing detailed information on existing VxVM objects (such as disk and volume attributes) and their relationships. Each disk group has an administrator-assigned name and an internally defined unique ID. The disk group names bootdg (an alias for the boot disk group), defaultdg (an alias for the default disk group) and nodg (represents no disk group) are reserved.

disk group ID

A unique identifier used to identify a disk group.

disk ID

A universally unique identifier that is given to each disk and can be used to identify the disk, even if it is moved.

disk media name

An alternative term for a disk name.

disk media record

A configuration record that identifies a particular disk, by disk ID, and gives that disk a logical (or administrative) name.

disk name

A logical or administrative name chosen for a disk that is under the control of VxVM, such as disk03. The term disk media name is also used to refer to a disk name.


The process by which any link that exists between two VxVM objects is removed. For example, dissociating a subdisk from a plex removes the subdisk from the plex and adds the subdisk to the free space pool.

dissociated plex

A plex dissociated from a volume.

dissociated subdisk

A subdisk dissociated from a plex.

distributed lock manager

A lock manager that runs on different systems in a cluster, and ensures consistent access to distributed resources.

enabled path

A path to a disk that is available for I/O.


A process that converts existing partitions on a specified disk to volumes.

Encapsulation is not supported on the AIX platform.


See disk enclosure.

enclosure-based naming

See device name.

fabric mode disk

A disk device that is accessible on a Storage Area Network (SAN) via a Fibre Channel switch.


A fast resynchronization feature that is used to perform quick and efficient resynchronization of stale mirrors, and to increase the efficiency of the snapshot mechanism.

Fibre Channel

A collective name for the fiber optic technology that is commonly used to set up a Storage Area Network (SAN).

file system

A collection of files organized together into a structure. The UNIX file system is a hierarchical structure consisting of directories and files.

free space

An area of a disk under VxVM control that is not allocated to any subdisk or reserved for use by any other VxVM object.

free subdisk

A subdisk that is not associated with any plex and has an empty putil[0] field.


A string that identifies a host to VxVM. The hostid for a host is stored in its volboot file, and is used in defining ownership of disks and disk groups.


A technique of automatically restoring redundancy and access to mirrored and RAID-5 volumes when a disk fails. This is done by relocating the affected subdisks to disks designated as spares and/or free space in the same disk group.


Refers to devices that can be removed from, or inserted into, a system without first turning off the power supply to the system.

initiating node

The node on which the system administrator is running a utility that requests a change to VxVM objects. This node initiates a volume reconfiguration.


The common name for an unintelligent disk array which may, or may not, support the hot-swapping of disks. The name is derived from "just a bunch of disks."

log plex

A plex used to store a RAID-5 log. The term log plex may also be used to refer to a Dirty Region Logging plex.

log subdisk

A subdisk that is used to store a dirty region log.

master node

A node that is designated by the software to coordinate certain VxVM operations in a cluster. Any node is capable of being the master node.

mastering node

The node to which a disk is attached. This is also known as a disk owner.


A duplicate copy of a volume and the data therein (in the form of an ordered collection of subdisks). Each mirror consists of one plex of the volume with which the mirror is associated.


A layout technique that mirrors the contents of a volume onto multiple plexes. Each plex duplicates the data stored on the volume, but the plexes themselves may have different layouts.


Where there are multiple physical access paths to a disk connected to a system, the disk is called multipathed. Any software residing on the host, (for example, the DMP driver) that hides this fact from the user is said to provide multipathing functionality.


One of the hosts in a cluster.

node abort

A situation where a node leaves a cluster (on an emergency basis) without attempting to stop ongoing operations.

node join

The process through which a node joins a cluster and gains access to shared disks.

Non-Persistent FastResync

A form of FastResync that cannot preserve its maps across reboots of the system because it stores its change map in memory.


An entity that is defined to and recognized internally by VxVM. The VxVM objects are: volume, plex, subdisk, disk, and disk group. There are actually two types of disk objects—one for the physical aspect of the disk and the other for the logical aspect.


A calculated value that can be used to reconstruct data after a failure. While data is being written to a RAID-5 volume, parity is also calculated by performing an exclusive OR (XOR) procedure on data. The resulting parity is then written to the volume. If a portion of a RAID-5 volume fails, the data that was on that portion of the failed volume can be recreated from the remaining data and the parity.

parity stripe unit

A RAID-5 volume storage region that contains parity information. The data contained in the parity stripe unit can be used to help reconstruct regions of a RAID-5 volume that are missing because of I/O or disk failures.


The standard division of a physical disk device, as supported directly by the operating system and disk drives.


When a disk is connected to a host, the path to the disk consists of the HBA (Host Bus Adapter) on the host, the SCSI or fibre cable connector and the controller on the disk or disk array. These components constitute a path to a disk. A failure on any of these results in DMP trying to shift all I/O for that disk onto the remaining (alternate) paths.


In the case of disks which are not multipathed by vxdmp, VxVM will see each path as a disk. In such cases, all paths to the disk can be grouped. This way only one of the paths from the group is made visible to VxVM.

Persistent FastResync

A form of FastResync that can preserve its maps across reboots of the system by storing its change map in a DCO volume on disk).

persistent state logging

A logging type that ensures that only active mirrors are used for recovery purposes and prevents failed mirrors from being selected for recovery. This is also known as kernel logging.

physical disk

The underlying storage device, which may or may not be under VxVM control.


A plex is a logical grouping of subdisks that creates an area of disk space independent of physical disk size or other restrictions. Mirroring is set up by creating multiple data plexes for a single volume. Each data plex in a mirrored volume contains an identical copy of the volume data. Plexes may also be created to represent concatenated, striped and RAID-5 volume layouts, and to store volume logs.

primary path

In Active/Passive disk arrays, a disk can be bound to one particular controller on the disk array or owned by a controller. The disk can then be accessed using the path through this particular controller.

private disk group

A disk group in which the disks are accessed by only one specific host in a cluster.

private region

A region of a physical disk used to store private, structured VxVM information. The private region contains a disk header, a table of contents, and a configuration database. The table of contents maps the contents of the disk. The disk header contains a disk ID. All data in the private region is duplicated for extra reliability.

public region

A region of a physical disk managed by VxVM that contains available space and is used for allocating subdisks.


A Redundant Array of Independent Disks (RAID) is a disk array set up with part of the combined storage capacity used for storing duplicate information about the data stored in that array. This makes it possible to regenerate the data if a disk failure occurs.

read-writeback mode

A recovery mode in which each read operation recovers plex consistency for the region covered by the read. Plex consistency is recovered by reading data from blocks of one plex and writing the data to all other writable plexes.

root configuration

The configuration database for the root disk group. This is special in that it always contains records for other disk groups, which are used for backup purposes only. It also contains disk records that define all disk devices on the system.

root disk

The disk containing the root file system. This disk may be under VxVM control.

root file system

The initial file system mounted as part of the UNIX kernel startup sequence.

root partition

The disk region on which the root file system resides.

root volume

The VxVM volume that contains the root file system, if such a volume is designated by the system configuration.


The ability to place the root file system and the swap device under VxVM control. The resulting volumes can then be mirrored to provide redundancy and allow recovery in the event of disk failure.

Rootability is not supported on the AIX platform.

secondary path

In Active/Passive disk arrays, the paths to a disk other than the primary path are called secondary paths. A disk is supposed to be accessed only through the primary path until it fails, after which ownership of the disk is transferred to one of the secondary paths.


A unit of size, which can vary between systems. Sector size is set per device (hard drive, CD-ROM, and so on). Although all devices within a system are usually configured to the same sector size for interoperability, this is not always the case.

A sector is commonly 512 bytes.

shared disk group

A disk group in which access to the disks is shared by multiple hosts (also referred to as a cluster-shareable disk group).

shared volume

A volume that belongs to a shared disk group and is open on more than one node of a cluster at the same time.

shared VM disk

A VM disk that belongs to a shared disk group in a cluster.

slave node

A node that is not designated as the master node of a cluster.


The standard division of a logical disk device. The terms partition and slice are sometimes used synonymously.


A point-in-time copy of a volume (volume snapshot) or a file system (file system snapshot).


A layout technique that permits a volume (and its file system or database) that is too large to fit on a single disk to be configured across multiple physical disks.

sparse plex

A plex that is not as long as the volume or that has holes (regions of the plex that do not have a backing subdisk).

Storage Area Network (SAN)

A networking paradigm that provides easily reconfigurable connectivity between any subset of computers, disk storage and interconnecting hardware such as switches, hubs and bridges.


A set of stripe units that occupy the same positions across a series of columns.

stripe size

The sum of the stripe unit sizes comprising a single stripe across all columns being striped.

stripe unit

Equally-sized areas that are allocated alternately on the subdisks (within columns) of each striped plex. In an array, this is a set of logically contiguous blocks that exist on each disk before allocations are made from the next disk in the array. A stripe unit may also be referred to as a stripe element.

stripe unit size

The size of each stripe unit. The default stripe unit size is 64KB. The stripe unit size is sometimes also referred to as the stripe width.


A layout technique that spreads data across several physical disks using stripes. The data is allocated alternately to the stripes within the subdisks of each plex.


A consecutive set of contiguous disk blocks that form a logical disk segment. Subdisks can be associated with plexes to form volumes.

swap area

A disk region used to hold copies of memory pages swapped out by the system pager process.

swap volume

A VxVM volume that is configured for use as a swap area.


A set of configuration changes that succeed or fail as a group, rather than individually. Transactions are used internally to maintain consistent configurations.

volboot file

A small file that is used to locate copies of the boot disk group configuration. The file may list disks that contain configuration copies in standard locations, and can also contain direct pointers to configuration copy locations. The volboot file is stored in a system-dependent location.

VM disk

A disk that is both under VxVM control and assigned to a disk group. VM disks are sometimes referred to as VxVM disks or simply disks.


A virtual disk, representing an addressable range of disk blocks used by applications such as file systems or databases. A volume is a collection of from one to 32 plexes.

volume configuration device

The volume configuration device (/dev/vx/config) is the interface through which all configuration changes to the volume device driver are performed.

volume device driver

The driver that forms the virtual disk drive between the application and the physical device driver level. The volume device driver is accessed through a virtual disk device node whose character device nodes appear in /dev/vx/rdsk, and whose block device nodes appear in /dev/vx/dsk.


The VxVM configuration daemon, which is responsible for making changes to the VxVM configuration. This daemon must be running before VxVM operations can be performed.