Sun Microsystems StorEdge 6920 System, Getting Started Manual

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Chapter 3 Configuring the System 63
What Is a Snapshot?
The Sun StorEdge Data Snapshot software allows you to make non-disruptive point-
in-time copies of a volume’s data. The snapshot image can then be mounted to
another server, enabling the primary volume to continue servicing the application.
Each snapshot can be accessed independently by other applications. For example, a
snapshot can be mounted on a backup server, which will back up the data to tape.
A snapshot is a copy of the data on a volume at the moment that you create the
snapshot. This snapshot implementation uses the copy-on-write, or dependent copy,
technology. For detailed information about the snapshot technology, see the online
help.
Note – A snapshot of data is not suitable for failure recovery. You should continue
to use offline backup methods to create full-volume backup copies of your data.
The system stores snapshots in an area on the primary volume called snapshot reserve
space. The snapshot reserve space is configured to take data from a primary volume
(the application’s volume) and store up to eight snapshots and associated metadata.
Snapshot Planning Considerations
Before you can create a snapshot, the volume must have snapshot reserve space. You
can allocate the snapshot reserve space when you first create the volume or when
you first request a snapshot of the volume. In either case, the New Snapshot wizard
guides you through the process.
After you allocate the snapshot reserve space, you cannot modify it, so it is
important to choose the proper size. The advantage to allocating the snapshot
reserve space at the time you create the volume is that you can be sure there is
sufficient free space for the snapshots. When you allocate the snapshot reserve space
at a later time, the amount of free space you need might not be available. If you do
not allocate enough snapshot reserve space, you risk running out of storage space
before a snapshot can be completed, thereby corrupting the snapshot data and
making previous snaphots unusable. In addition, if the snapshot reserve space you
created is too small, you must delete existing snapshots as required to make space
for new ones.
The proper size of the snapshot reserve space depends on the following factors:
■
The number of snapshots to be taken
The more snapshots you take, the more space is required for the snapshot reserve
space. Specify the size of the snapshot reserve space as a percentage of the
primary volume. Depending on the number of snapshots you want, specify from
1% to 800% of the primary volume’s size.