Categorized | Volumes

Veritas Volume Manager – vxdiskadm

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About Veritas volume manager :

Veritas Volume Manager is used to manage disk storage spread over an array of disks.

The main features of volume manager are following

1. Allows creation of logical volumes spanning over multiple disks. This overcomes the physical limit of the disk .
2. Provides high availability storage solutions through RAID ,Mirroring of disks .
3. Provides fail over features by providing transferable disk group ownership between systems.
4. Dynamic reconfiguration of disk storage in an online system state. what is veritas volume manager .

The following article describes the volume manager objects and configuration of these objects using a text menu based utility called vxdiskadm .

Table of Contents :
1. Volume Manager Objects
1.1 Disks
1.2 Disk groups
1.3 Volume Manager disks
1.4 Subdisks
1.5 Plexes
1.6 Volumes
1.7 Volume Manger Objects & their Relationship

2. Volume Manager Configuration ( options menu)
2.1 Add or initialize one or more disks
2.2 Encapsulate one or more disks
2.3 Remove a disk
2.4 Remove a disk for replacement
2.5 Replace a failed or removed disk
2.6 Mirror volumes on a disk
2.7 Move volumes from a disk
2.8 Enable access to (import) a disk group
2.9 Remove access to (deport) a disk group
2.10 Enable (online) a disk device
2.11 Disable (offline) a disk device
2.12 Mark a disk as a spare for a disk group
2.13 Turn off the spare flag on a disk

1.0 Volume Manager Objects
Disks
Disks are referred in volume manager by two terms – device name and disk name . The device name specifies controller , target id and slice of the disk . Disk name is the common name given to the device name as an easy to remember name .

For example device name c2t3d0s2 represents controller number 2 , target id 3 , disk group 0 and slice 2 and disk01 may be its disk name . While device name is system dependent based on controller and disk id the disk name is user defined .

Disk groups
* A disk group is a collection of volume manager disks grouped together to hold the data . All the configuration changes made to a disk group are applied to the disks in that disk group only.
* Volume Manager objects cannot span disk groups i.e. all the operations on a particular disk group remains confined to that particular group .
* Disk groups enable high availability as these can be shared by two or more hosts but can be accessed by only one host at a time. In two hosts and a shared storage situation one host can take over the ownership of the disk groups and drives in case other host fails.

Volume Manager disks
* Adding physical disks to the volume manager results in creation of public and private region in the disk by the volume manager .The public region is the disk space available for volume space and the private region stores the configuration information.
* A Volume Manager disks are created from the public region of a physical disk that is under Volume Manager control. Each volume manager disk corresponds to one physical disk.
* A volume manager disk is given a disk media name when it is added to a disk group which can be default or unique user defined..
* Once a volume manager disk is assigned a disk media name, the disk is no longer referred to by its physical address of c#t#d#. The physical address of c#t#d# becomes known as the disk access record.

Subdisks
* A subdisk is a subsection of a disk’s public region and is the smallest unit of storage in Volume Manager.
* A subdisk is defined by an offset and a length in sectors on a volume manager disk.
* A volume manager disk can contain multiple subdisks but subdisks cannot overlap or share the same portions of a volume manager disk.
* volume manager disk space that is not reserved or that is not part of a subdisk is free space. You can use free space to create new subdisks.

A subdisk is similar to a partition but with following differences :
* The maximum number of partitions to a disk is eight.
* There is no theoretical limit to number of subdisks that can be attached to a single plex, but it has been limited to a default value of 4096. If required, this default can be changed, using the vol_subdisk_num tunable parameter.

Plexes

* A plex is a structured or ordered collection of subdisks that represents one copy of the data in a volume. A plex consists of one or more subdisks located on one or more physical disks.
* A plex is also called a mirror. The terms plex and mirror can be used interchangeably, even though a plex is only one copy of the data. The terms “mirrored” or “mirroring” imply two or more copies of data.
* The length of a plex is determined by the last block that can be read or written on the last subdisk in the plex.
* The default naming convention for plexes in a volume is volumename-##. The default plex name consists of the volume name, a hyphen, and a two-digit number

Volumes

* A volume is composed of one or more plexes not restricted by the physical size of the disk.
* A volume can span across multiple disks.
* Volume Manager uses the default naming convention vol## for volumes, where ## represents a two-digit number but can be user defined as per requirement.

Volume Manager Objects and Their Relationship

2.0 Volume Manager Configuration

Volume configuration consists of adding two or more disks to form disk group and create volume/s from this disk group. File system can be created on these volumes or these can be accessed as raw devices for some database applications. First step in volume manager configuration consists of adding and initializing disks under volume manager which creates a public region which is bulk of disk space available for volume space .Private region which is generally of a few megabytes stores the disk configuration information .

Most of the volume manager operation related to disks & volume can be performed by a character based menu utility call vxdiskadm

Following pages describes the use of vxddiskadm to configure and manage volume manager.

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