VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
Volume Layouts in VxVM
Chapter 118
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
Volumes” on page 36 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.