Sun Microsystems StorageTek 6140 Administration Manual page 90

The capacity needed for the snapshot reserve volume varies, depending on the
frequency and size of I/O writes to the base volume and how long you need to keep
the snapshot volume. In general, choose a large capacity for the reserve volume if
you intend to keep the snapshot volume for a long period of time or if you anticipate
heavy I/O activity, which will cause a large percentage of data blocks to change on
the base volume during the life of the snapshot volume. Use historical performance
to monitor data or other operating system utilities to help you determine typical I/O
activity on the base volume.
When the snapshot reserve volume reaches a specified capacity threshold, you are
given a warning. You set this threshold when you create a snapshot volume. The
default threshold level is 50 percent.
If you receive a warning and determine that the snapshot reserve volume is in
danger of filling up before you have finished using the snapshot volume, you can
increase its capacity by navigating to the Snapshot Details page and clicking
Expand. If the snapshot reserve volume fills up before you have finished using the
snapshot, the snapshot failure handling conditions specify the action that will be
taken.
When you create a snapshot volume, you can allocate as large a snapshot reserve
volume as you have space for.
Use the following information to determine the appropriate capacity of the snapshot
reserve volume:
A snapshot reserve volume cannot be smaller than 8 megabytes.
The amount of write activity to the base volume after the snapshot volume has
been created dictates how large the snapshot reserve volume needs to be. As the
amount of write activity to the base volume increases, the number of original data
blocks that need to be copied from the base volume to the snapshot reserve
volume also increases.
The estimated life expectancy of the snapshot volume contributes to determining
the appropriate capacity of the snapshot reserve volume. If the snapshot volume
is created and remains enabled for a long period of time, the snapshot reserve
volume runs the risk of reaching its maximum capacity.
The amount of management overhead required on the snapshot reserve volume
for storage of snapshot volume data contributes to determining the appropriate
capacity of the snapshot reserve volume. The amount of management overhead
actually required is fairly small, and can be calculated with the simple formula
that is presented later in this topic.
There is not necessarily a one-to-one correlation between the number of data
blocks that change on the base volume and the amount of data stored in the
snapshot reserve volume. Depending on the location of data blocks that need to
be copied, performance factors might dictate that the controller copy over a full
74 Sun StorageTek Array Administration Guide • May 2006