Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 1, Understanding VERITAS Volume Manager

Volume Layouts in VxVM

A VxVM virtual device is defined by a volume. A volume has a layout defined by the

association of a volume to one or more plexes, each of which map to subdisks. The volume

presents a virtual device interface that is exposed to other applications for data access.

These logical building blocks re-map the volume address space through which I/O is

re-directed at run-time.

Different volume layouts each provide different levels of storage service. A volume layout

can be configured and reconfigured to match particular levels of desired storage service.

Implementation of Non-Layered Volumes

In a non-layered volume, a subdisk is restricted to mapping directly to a VM disk. This

allows the subdisk to define a contiguous extent of storage space backed by the public

region of a VM disk. When active, the VM disk is directly associated with an underlying

physical disk. The combination of a volume layout and the physical disks therefore

determines the storage service available from a given virtual device.

Implementation of Layered Volumes

A layered volume is constructed by mapping its subdisks to underlying volumes. The

subdisks in the underlying volumes must map to VM disks, and hence to attached

physical storage.

Layered volumes allow for more combinations of logical compositions, some of which

may be desirable for configuring a virtual device. Because permitting free use of layered

volumes throughout the command level would have resulted in unwieldy administration,

some ready-made layered volume configurations are designed into VxVM. See “Layered

Vol um es ” on page 34 for more information.

These ready-made configurations operate with built-in rules to automatically match

desired levels of service within specified constraints. The automatic configuration is done

on a “best-effort” basis for the current command invocation working against the current

configuration.

To achieve the desired storage service from a set of virtual devices, it may be necessary to

include an appropriate set of VM disks into a disk group, and to execute multiple

configuration commands.

To the extent that it can, VxVM handles initial configuration and on-line re-configuration

with its set of layouts and administration interface to make this job easier and more

deterministic.