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Virtual objects

Virtual objects in VxVM include the following:

The connection between physical objects and VxVM objects is made when you place a physical disk under VxVM control.

After installing VxVM on a host system, you must bring the contents of physical disks under VxVM control by collecting the VM disks into disk groups and allocating the disk group space to create logical volumes.

To bring the physical disk under VxVM control, the disk must not be under LVM control.

See the Veritas Volume Manager Migration Guide.

Bringing the contents of physical disks under VxVM control is accomplished only if VxVM takes control of the physical disks and the disk is not under control of another storage manager such as LVM.

VxVM creates virtual objects and makes logical connections between the objects. The virtual objects are then used by VxVM to do storage management tasks.

The vxprint command displays detailed information about the VxVM objects that exist on a system.

See "Displaying volume information" on page 258.

See the vxprint(1M) manual page.

Combining virtual objects in VxVM

VxVM virtual objects are combined to build volumes. The virtual objects contained in volumes are VM disks, disk groups, subdisks, and plexes. VxVM virtual objects are organized in the following ways:

Connection between objects in VxVM shows the connections between Veritas Volume Manager virtual objects and how they relate to physical disks.

Connection between objects in VxVM

Connection between objects in VxVM

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The disk group contains three VM disks which are used to create two volumes. Volume vol01 is simple and has a single plex. Volume vol02 is a mirrored volume with two plexes.

The various types of virtual objects (disk groups, VM disks, subdisks, plexes and volumes) are described in the following sections. Other types of objects exist in Veritas Volume Manager, such as data change objects (DCOs), and cache objects, to provide extended functionality.

Disk groups

A disk group is a collection of disks that share a common configuration, and which are managed by VxVM. A disk group configuration is a set of records with detailed information about related VxVM objects, their attributes, and their connections. A disk group name can be up to 31 characters long.

See "VM disks" on page 27.

In releases before VxVM 4.0, the default disk group was rootdg (the root disk group). For VxVM to function, the rootdg disk group had to exist and it had to contain at least one disk. This requirement no longer exists, and VxVM can work without any disk groups configured (although you must set up at least one disk group before you can create any volumes of otherVxVM objects).

See "Creating and administering disk groups" on page 159.

You can create additional disk groups when you need them. Disk groups allow you to group disks into logical collections. A disk group and its components can be moved as a unit from one host machine to another.

See "Reorganizing the contents of disk groups" on page 189.

Volumes are created within a disk group. A given volume and its plexes and subdisks must be configured from disks in the same disk group.

VM disks

When you place a physical disk under VxVM control, a VM disk is assigned to the physical disk. A VM disk is under VxVM control and is usually in a disk group. Each VM disk corresponds to one physical disk. VxVM allocates storage from a contiguous area of VxVM disk space.

A VM disk typically includes a public region (allocated storage) and a small private region where VxVM internal configuration information is stored.

Each VM disk has a unique disk media name (a virtual disk name). You can either define a disk name of up to 31 characters, or allow VxVM to assign a default name that takes the form diskgroup##, where diskgroup is the name of the disk group to which the disk belongs.

See "Disk groups" on page 27.

VM disk example shows a VM disk with a media name of disk01 that is assigned to the physical disk, devname.

VM disk example

VM disk example

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Subdisks

A subdisk is a set of contiguous disk blocks. A block is a unit of space on the disk. VxVM allocates disk space using subdisks. A VM disk can be divided into one or more subdisks. Each subdisk represents a specific portion of a VM disk, which is mapped to a specific region of a physical disk.

The default name for a VM disk is diskgroup## and the default name for a subdisk is diskgroup##-##, where diskgroup is the name of the disk group to which the disk belongs.

See "Disk groups" on page 27.

Subdisk example shows disk01-01 is the name of the first subdisk on the VM disk named disk01.

Subdisk example

Subdisk example

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A VM disk can contain multiple subdisks, but subdisks cannot overlap or share the same portions of a VM disk.

Example of three subdisks assigned to one VM Disk shows a VM disk with three subdisks, which are assigned from one physical disk.

Example of three subdisks assigned to one VM Disk

Example of three subdisks assigned to one VM Disk

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Any VM disk space that is not part of a subdisk is free space. You can use free space to create new subdisks.

VxVM supports the concept of layered volumes in which subdisks can contain volumes.

See "Layered volumes" on page 50.

Plexes

VxVM uses subdisks to build virtual objects called plexes. A plex consists of one or more subdisks located on one or more physical disks.

Example of a plex with two subdisks shows an example of a plex with two subdisks.

Example of a plex with two subdisks

Example of a plex with two subdisks

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You can organize data on subdisks to form a plex by using the following methods:

Concatenation, striping (RAID-0), mirroring (RAID-1) and RAID-5 are additional types of volume layout.

See "Volume layouts in VxVM" on page 33.

Volumes

A volume is a virtual disk device that appears to applications, databases, and file systems like a physical disk device, but does not have the physical limitations of a physical disk device. A volume consists of one or more plexes, each holding a copy of the selected data in the volume. Due to its virtual nature, a volume is not restricted to a particular disk or a specific area of a disk. The configuration of a volume can be changed by using VxVM user interfaces. Configuration changes can be accomplished without causing disruption to applications or file systems that are using the volume. For example, a volume can be mirrored on separate disks or moved to use different disk storage.

VxVM uses the default naming conventions of vol## for volumes and vol##-## for plexes in a volume. For ease of administration, you can choose to select more meaningful names for the volumes that you create.

A volume may be created under the following constraints:

You can use the Veritas Intelligent Storage Provisioning (ISP) feature to create and administer application volumes. These volumes are very similar to the traditional VxVM volumes that are described in this chapter. However, there are significant differences between the functionality of the two types of volume that prevent them from being used interchangeably.

See the Veritas Storage Foundation Intelligent Storage Provisioning Administrator's Guide.

Example of a volume with one plex shows a volume vol01 with a single plex.

Example of a volume with one plex

Example of a volume with one plex

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The volume vol01 has the following characteristics:

Example of a volume with two plexes shows a mirrored volume, vol06, with two data plexes.

Example of a volume with two plexes

Example of a volume with two plexes

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Each plex of the mirror contains a complete copy of the volume data.

The volume vol06 has the following characteristics:

See "Mirroring (RAID-1)" on page 41.