IBM N Series, Hardware Manual

Page: 214 / 368

194
IBM System Storage N series Hardware Guide
To return to reconciling usable storage to raw storage, this example suggests that 65% or
55% of raw capacity is available for storing user data. The percentage depends on how you
classify the snapshot reserve. In general, larger environments tend to result in a higher ratio
of raw to usable capacity.
14.2.2 Other effects of Snapshot
It is important to understand the potential effect of creating and retaining Snapshots, on the N
series controller and any associated servers and applications. Also, the Snapshots must be
coordinated with the attached servers and applications to ensure data integrity.
The effect of Snapshots is determined by the following factors:
򐂰 N series controller:
– Negligible effect on the performance of the controller
The N series snapshots use a redirect-on-write design. This design avoids most of the
performance effect that is normally associated with Snapshot creation and retention
(as seen in traditional copy-on-write snapshots on other platforms).
– Incremental capacity is required to retain any changes
Snapshot technology optimizes storage because only changed blocks are retained. For
file access, the change rate is typically in the 1 - 5% range. For database applications,
it might be similar. However, in some cases it might be as high as 100%.
򐂰 Server (SAN-attached):
– Minor effect on the performance of the server when the Snapshot is created (to ensure
file system and LUN consistency).
– Negligible ongoing effect on performance to retain the Snapshots
򐂰 Application (SAN or NAS attached):
– Minor effect on the performance of the application when the snapshot is created (to
ensure data consistency). This effect depends on the snapshot frequency. Once per
day, or multiple times per day might be acceptable, but more frequent Snapshots can
have an unacceptable effect on application performance.
– Negligible ongoing effect on performance to retain the Snapshots
򐂰 Workstation (NAS attached):
– No effect on the performance of the workstation. Frequent Snapshots are possible
because the NAS file system consistency is managed by the N series controller.
– Negligible ongoing effect on performance to retain the Snapshots
Attention: When the N series gateway is introduced in a pre-existing environment, the
final usable capacity is different from that available on the external disk system before
being virtualized.