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

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Recommended practices
The following practices are recommended when Version 2.2 volume groups are configured
with snapshots:
• The size of the configuration file is bigger for Version 2.2 volume groups. Set the location of the
configuration file in the variable LVMCONFPATH_NON_BOOT in the /etc/lvmrc file and ensure
that sufficient space is allocated to this location to hold the large configuration files.
• Based on the type of application, choose the unshare unit based on the guidelines mentioned in
Creating a version 2.2 (or higher) volume group
• Do not override the automatic backup of configuration file done by vgchange.
• When you back up the configuration file of a volume group with snapshots, ensure that there are no
I/Os in progress so that the configuration file has latest sharing information.
• When you use the configuration file of a Version 2.2 volume group with snapshots to restore the
configuration on all available physical volumes of a volume group, ensure that the configuration file
is updated with the data sharing information of the snapshots to prevent data corruption on
the snapshots.
• To ensure correctness of the data in a snapshot logical volume, HP recommends that applications
using the logical volume be quiesced prior to initiating the snapshot creation.
• For logical volumes on a snapshot tree, set the timeout to a larger value to accommodate the CBW
I/Os induced by a write I/O on any logical volume on the snapshot tree. For more information, see
Changing the attributes of a snapshot logical volume
• When you import a volume group with snapshots, to retain the user-defined names of the snapshot
logical volumes, use a map file generated by a corresponding vgexport operation. For more
information, see
Importing volume groups with snapshots
• In very low system memory conditions, deletion of a single snapshot can hang indefinitely for
memory to become available
. Ensure that sufficient memory is available during deletion of a single
snapshot
that requires data to be copied to its predecessor. If the lvremove command hangs in these
cases, increase
the system memory or free some existing system memory to proceed with the snapshot
deletion.
• To restore data on the original logical volume, if you copy the entire contents of its space-efficient
snapshot, the shared data on the space-efficient snapshot is unshared. Before performing the data
restore, ensure that enough extents are pre-allocated to the space efficient snapshot or selectively
copy only the unshared data of the snapshot to its original logical volume. Alternatively, if you want
to restore the entire contents of the snapshot to its original logical volume, use a fully-
allocated snapshot.
Error messages
When snapshots are configured, under certain conditions, you might see the following
error messages:
• Error: Exceeds the maximum supported snapshot capacity (4194304 MB)
for the specified volume group /dev/vg01.
During snapshot creation, lvcreate reports that the creation of a new snapshot for the specified
original logical volume cannot be performed as the resulting current snapshot capacity will exceed
the maximum supported snapshot capacity for this volume group. This can occur when the size of
the logical volume for which the snapshot creation is requested is more than the total snapshot
capacity remaining in the volume group. Use vgmodify to increase the maximum volume group