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

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The first instance of a write to the logical volume with snapshot takes more time and over a period of
time, the time taken equals to that of a mirrored logical volume with no snapshots, and then reduces
to that of a non-mirrored logical volume with no snapshots.
Configuration file size
Because of the increased metadata size for a Version 2.2 volume group, the configuration file size
increases each time a snapshot is created. When the volume has no snapshot logical volumes, the
size of the configuration file is comparable to that of a Version 2.0 or 2.1 volume group of the same
configuration. When a snapshot is created, information related to the snapshot and its data sharing
relationship is stored in the configuration file, which increases the size of the configuration file.
The increase in size is different for a space-efficient snapshot and a fully allocated snapshot for the
same snapshot capacity. For a space-efficient snapshot, this increase in size depends on the snapshot
capacity. For a fully-allocated snapshot, this increase in size depends on the snapshot capacity and
the extent size.
The following table summarizes the average increase in the size of the configuration file for
each1000 GB of snapshot capacity (for a fully allocated snapshot, the increase is given for two
different extent sizes).
Unshare unit (in KB)
Avg. increase in
configuration file size for a
space efficient snapshot
(in KB)
Avg. increase in
configuration file size for a
fully allocated snapshot with
8 MB extent size(in KB)
Avg. increase in
configuration file size for a
fully allocated snapshot with
32 MB extent size(in KB)
512 252.9 1253.5 503.2
1024 126.3 1125.5 375.2
2048 64.4 1065.5 315.2
4096 32.7 1033.5 283.2
The increase in size of the configuration file is less for a space-efficient snapshot creation because at
the point of snapshot creation, entire data is shared with the original logical volume and no physical
extents are set aside on disk for the snapshot. Over time, as unsharing of a space efficient snapshot
progresses, the snapshot gets its own copy of data and the extent mapping on disk is updated to
include the new physical extents. When the entire space-efficient snapshot is unshared, the increase in
size of configuration file is equivalent to the size of a fully-allocated snapshot of similar configuration.
The configuration file does not get updated automatically after each data unshare operation. So the
increase is the file size is seen only when the configuration file is backed up explicitly using
vgcfgbackup or during a configuration change that takes automatic backup of the file. If you
choose to compress the configuration file to reduce the file size, a subsequent configuration operation
that performs automatic backup of the configuration file recreates the file (uncompressed file).
Note
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
the latest sharing information.