VxVM System Administrator's Guide

Glossary

associate

The process of establishing a relationship between Volume Manager 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.

attached

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

block

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

boot disk

A disk used for booting purposes. This disk may be under VxVM control.

column

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.

concatenation

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

configuration database

A set of records containing detailed information on existing Volume Manager objects (such as disk and volume attributes). A single copy of a configuration database is called a configuration copy.

data stripe

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

detached

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 c0b0t0d0s0. The c#b#t#d#s# syntax identifies the controller, target address, disk, and partition.

Dirty Region Logging

The procedure by which the Volume Manager monitors and logs modifications to a plex. A bitmap of changed regions is kept in an associated subdisk called a log subdisk.

disk

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

The name used to access a physical disk, such as c0b0t0d0. The c#b#t#d#s# syntax identifies the controller, target address, disk, and partition. The term device name can also be used to refer to the disk access 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 the Volume Manager 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 group

A collection of disks that share a common configuration. A disk group configuration is a set of records containing detailed information on existing Volume Manager 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 root disk group (rootdg) is a special private disk group that always exists.

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

A logical or administrative name chosen for the disk, such as disk03. The term disk name is also used to refer to the disk media name.

disk media record

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

dissociate

The process by which any link that exists between two Volume Manager 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.

encapsulation

A process that converts existing partitions on a specified disk to volumes. If any partitions contain file systems, /etc/vfstab entries are modified so that the file systems are mounted on volumes instead.

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 Volume Manager object.

free subdisk

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

hostid

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

hot-relocation

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.

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. See Dirty Region Logging.

mirror

A duplicate copy of a volume and the data therein (in the form of an ordered collection of subdisks). Each mirror is one copy of the volume with which the mirror is associated. The terms mirror and plex can be used synonymously.

mirroring

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.

object

An entity that is defined to and recognized internally by the Volume Manager. 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.

parity

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.

partition

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

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 Volume Manager control.

plex

A duplicate copy of a volume and the data therein (in the form of an ordered collection of subdisks). Each plex is one copy of the volume with which the plex is associated. The terms mirror and plex can be used synonymously.

private region

A region of a physical disk used to store private, structured Volume Manager 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 the Volume Manager that contains available space and is used for allocating subdisks.

RAID

A Redundant Array of Inexpensive 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 disk group

A special private disk group that always exists on the system. The root disk group is named rootdg.

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.

rootability

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

sector

A unit of size, which can vary between systems. A sector is commonly 512 bytes.

slice

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

spanning

A layout technique that permits a volume (and its file system or database) too large to fit on a single disk to span 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 don't have a backing subdisk).

stripe

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 32 sectors (16K). A stripe unit size has also historically been referred to as a stripe width.

striping

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.

subdisk

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.

transaction

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 root configuration. The file may list disks that contain configuration copies in standard locations, and can also contain direct pointers to configuration copy locations. volboot is stored in a system-dependent location.

VM disk

A disk that is both under Volume Manager control and assigned to a disk group. VM disks are sometimes referred to as Volume Manager disks or simply disks. In the Visual Administrator, VM disks are represented iconically as cylinders labeled D.

volume

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.

volume event log

The volume event log device (/dev/vx/event) is the interface through which volume driver events are reported to the utilities.

vxconfigd

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