Removing A Dump Logical Volume; Creating And Administering Snapshot Logical Volumes; Types Of Snapshots - HP -UX 11i Administrator's Manual

After creating a logical volume to be used as a dump device, use the lvlnboot command with
the -d option to update the dump information used by LVM. If you created a logical volume /dev/
vg00/lvol2 for use as a dump area, update the boot information by entering the following:
# lvlnboot -d /dev/vg00/lvol2

Removing a Dump Logical Volume

To discontinue the use of a currently configured logical volume as a dump device, use the lvrmboot
command with the -d option. For example:
# lvrmboot -d /dev/vg00/lvol2
You can then use the logical volume for other purposes. To remove the logical volume completely,
see "Removing a Logical Volume" (page

Creating and Administering Snapshot Logical Volumes

Beginning with the HP-UX 1 1i v3 March 2010 Update, LVM provides the ability to create snapshot
logical volumes. A snapshot represents a point-in-time image of a logical volume. LVM snapshots
let you:
Use snapshots to back up data on the logical volume without splitting the logical volume. This
reduces the space requirement.
Create snapshots faster than manually copying over multiple point-in-time copies of the logical
volume.
Create multiple snapshots of a logical volume. This enables you to have images of the logical
volume for multiple point in time without allocating space equivalent to the entire size of the
logical volume per copy.
Up to 255 snapshots can be created off a single logical volume. The logical volume (referred to
as the original logical volume) and all of its snapshots together form a snapshot tree and maintains
a successor/predecessor relationship with each other.
When created, a snapshot shares all its data with that of the original logical volume. The snapshot
gets a copy of its own data only when a write occurs (copy before write) onto itself or its successor.
This process is called data unsharing. The smallest unit for data unsharing is the volume group's
unshare unit, specified by the vgcreate -U option. See vgcreate(1M) for details.
Snapshots are by default read-only, but the user can choose to create writable snapshots. See
lvcreate(1M) and lvchange(1M) for more details.
For more information about snapshot logical volumes and snapshot trees, see the lvm(7) manpage
and the LVM Snapshot Logical Volumes white paper.

Types of Snapshots

Snapshots can be of two types: fully-allocated and space-efficient.
When a fully allocated snapshot is created, the number of extents required for the snapshot
is allocated immediately, just like for a normal logical volume. However, the data contained
in the original logical volume is not copied over to these extents. The copying of data occurs
through the data unsharing process.
When a space-efficient snapshot is created, the user is expected to specify the number of
extents that LVM needs to set aside to be used for unsharing in the future. These extents are
referred to as pre-allocated extents.
When an unshare operation on a snapshot can not find the required extents in the pre-allocated
extent pool, the snapshot will be marked as over-committed and inoperative. To avoid this,
after the snapshot creation, the number of pre-allocated extents may be increased using the
lvextend command with the -l or -L option. Refer to the lvcreate(1M) and lvextend(1M)
manpages for details..
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