34Understanding Veritas Volume Manager

Volume layouts in VxVM

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-layeredvolume, 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 volumes” on page 51.

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