HP StorageWorks 2000 Modular Smart Array Reference Guide (481599-003, August 2008)

Chapter 3 Managing Storage 105
The snapshot service uses the single copy-on-write function to capture only data
that has changed. That is, if a block is to be overwritten on the master volume, and
a snapshot depends on the existing data in the block being overwritten, the data is
copied from the master volume to the snap pool before the data is changed. All
snapshots that depend on the older data are able to access it from the same location
on the snap pool; this reduces the impact of snapshots on master volume writes. In
addition, only a single copy-on-write operation is performed on the master volume.
Maximum Number of Snapshots
Each storage system permits a maximum number of snapshots to be retained, as
determined by an installed license. For example, if the maximum number of
snapshots allowed on your system is four, when the fifth snapshot is taken, an error
message informs you that you have reached the maximum number of snapshots
allowed on your system. You can delete an existing snapshot and take another
snapshot if the size of the snapshot is within the limits of the snap pool threshold.
The maximum number of snapshots can be increased by adding a license. See
“Managing Licenses” on page 35 for more information.
Estimating Snap-Pool Size
To help you accurately set a snap pool’s size, consider the following:
■ For each master volume, determine:
■ What is the master volume size, and how much will master volume data
change between snapshots? The size of the master volume and the average
amount of data change should be factored into snap pool sizing. Each snapshot
requires space in the snap pool. The amount of space needed depends on the
interval between snapshots and the number of updates to the master volume. If
the interval between snapshots is long, the amount of data written to the snap
pool will likely be greater. If the interval between snapshots is short, the
likelihood of a large number of changes to the data is less.
■ How many snapshots will be retained? Determine the number of snapshots
that will be retained for each master volume snapped. If you intend to use the
volume-copy service, add one to this number.
■ How many snapshots will be modified? Snapshots can be mounted as read-
only or read-write. Determine if the snapshots for a given volume will be
mounted as read-write and actually written to.