HP-UX 11i v3 Using LVM Logical Volume Snapshots (September 2010)

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Abstract
Beginning with the HP-UX 11i v3 March 2010 Update release, LVM supports Version 2.2 volume
groups. This update also enables you to create snapshots of logical volumes belonging to Version 2.2
volume groups.
This white paper explains how to create and administer the snapshots, limitations when using
snapshots, and recommended practices.
Audience
The document is intended for system administrators, operators, and customers who want to create and
use snapshot logical volumes belonging to Version 2.2 volume groups. It is assumed that you have a
basic knowledge of LVM.
HP-UX LVM snapshot overview
A snapshot is a point-in-time (PIT) image of data. Typically, a snapshot is created almost
instantaneously and is immediately available for use by applications. While the original copy can
change because of writes made to it, the snapshot retains the data contained in the original copy at
the point in time when the snapshot was created.
The logical volume from which the snapshot is created is referred to as the original logical volume.
When created, a snapshot shares all of its data with the original logical volume. The snapshot gets a
copy of its own data only when a write occurs onto itself or the original logical volume. LVM supports
read-only and read-write snapshots. By default, a read-only snapshot is created unless explicitly
specified during snapshot creation.
Snapshots are a special type of logical volume and contribute to the limit of maximum number of
logical volumes in a volume group (for example, 2047 logical volumes for a Version 2.2 volume
group). The size of a snapshot logical volume is the same as the size of the original logical volume.
Snapshots inherit their allocation policies from the original logical volume unless specified explicitly.
However, the contiguous allocation policy is not supported for space-efficient snapshots. The
allocation policy of a space-efficient snapshot is set to default allocation when the original logical
volume is configured for contiguous allocation. For more information, see
Creating snapshots
Types of snapshot logical volumes
HP-UX LVM supports the following snapshots:
• Fully-allocated snapshots
• Space-efficient snapshots
Fully-allocated snapshots
When a fully-allocated snapshot is created, the number of extents or the total storage capacity
required for the snapshot is allocated immediately, just like for a normal logical volume. The data
contained in the original logical volume is not copied to these extents. The copying of data happens
on an on-demand basis as and when data on the original logical volume or snapshot logical
volume changes.
Space-efficient snapshots
Unlike a fully-allocated snapshot, when a space-efficient snapshot is created, all the extents of the
snapshot are not allocated immediately. The user has to explicitly specify the number of extents that