HP-UX 11i v3 Using LVM Logical Volume Snapshots (September 2010)
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LVM: WARNING: Snapshot LV (VG 128 0x000000, LV Number: 6) threshold value
reached. Please increase the number of pre-allocated extents for this
snapshot LV
You can easily identify the snapshot logical volume by the volume group and logical volume numbers
in the message. LVM also publishes an LVM_EV_SNAPLV_THRESHOLD_REACHED event.
HP recommends that you monitor the syslog for the message or subscribe to the event. When the
threshold is reached, increase the number of extents in the pre-allocated pool to prevent the snapshot
from becoming over-committed. For more information, see
Extending the pre-allocated extent pool
Beginning with the HP-UX 11i v3 September 2010 Update release, LVM provides a feature to
automatically increase the pre-allocated extents when the threshold value is reached and provides an
option to set the threshold value. For more information, see
Enabling the automatic increase of pre-
allocated extents
Note
Although you can increase the number of extents in the pre-allocated pool,
you cannot reduce them.
Data unsharing can occur on a read-only and read-write snapshot logical volume. On a read-only
snapshot logical volume, data unsharing can occur because of a write I/O on its successor. On a
read-write snapshot logical volume, data unsharing can occur because of a write on the snapshot or
its successor. In either case, if the snapshot is space efficient, HP recommends you monitor for the
threshold of number of extents in its pre-allocated extent pool. Also, in case of deletion of a single
snapshot, data might get unshared on the predecessor of the snapshot being deleted. If the
predecessor is a space efficient snapshot, HP recommends you monitor for the threshold of
number of extents in its pre-allocated extent pool. For more information,
see
Deleting logical volumes on a snapshot tree
If a logical volume has snapshots associated with it, there might be an increase in the latencies
associated with reads and writes on the original logical volume or the snapshot. The increased
latency is not incurred after the first write on a unshare unit. For more information on the performance
of I/Os on snapshot logical volumes, see
Performance numbers
Creating and administering snapshots
LVM supports the creation of snapshot logical volumes on Version 2.2 and higher volume groups. To
configure the unshare unit, a new -U option is introduced in the vgcreate command. The supported
values are 512 KB, 1024 KB, 2048 KB, and 4096 KB. If the –U option is not used when creating the
volume group, the unshare unit is set to the default value of 1024 KB. The unshare unit cannot be
greater than the size of the physical extent of the volume group. After this value is set, you cannot
change it.
The total capacity of the snapshots you can create in a volume group is referred to as the maximum
snapshot capacity. LVM provisions the maximum snapshot capacity to be equivalent to the maximum
volume group capacity specified when creating the volume group.
For example, to create a Version 2.2 volume group with the unshare unit as 512 KB, extent size
8 MB, and maximum snapshot capacity of 128 TB, enter the following command:
# vgcreate –V 2.2 –S 128t –s 8 –U 512 vg01 /dev/disk/disk40
LVM stores the information related to sharing relationship of the unshare units of each snapshot
logical volume in its metadata. The size of the metadata for a Version 2.2 volume group depends on
the size of unshare unit chosen at the volume group creation time. If the unshare unit is small