User Manual

Advanced Functions
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Phase 2: Configuration
5. Create volumes of the secondary volumes.
6. Select a secondary volume for each source volume (primary
volume).
7. Set up snapshot options, like overflow alert.
8. Build snapshot automation scripts and conduct trail runs (optionally).
Phase 3: Creating and Utilizing Snapshots (Manually or Scripted)
When taking a snapshot for a source volume, the tasks below are
required:
9. Stop all the write access to the LUN of the primary volume.
10. Flush the cached data on the memory of your host systems.
11. Create the snapshot volume.
12. Resume the write access to the LUN of the primary volume.
13. Export the snapshot volume to a LUN for your applications
(optionally)
14. Copy the data from the snapshot volume to other places
(optionally)
15. Delete the snapshot volumes (optionally) to avoid running out of
space on the secondary volumes.
The tasks in the phase 1 and 2 are done only once when you set up a
RAID system or when you create a new LUN. They could also be parts of
your RAID system reconfigurations. The tasks in phase 3 are very likely to
be repeated periodically when a snapshot is needed.
• Planning for the Secondary Volumes
When planning your storage resources, you have to reserve sufficient free
capacity for the secondary volumes. In addition to the space reserved for
the RAID controller to build lookup tables, the capacity reserved for the
secondary volumes depends on how much data could be modified on
the primary and snapshot volumes during the life time of the snapshot
volumes. If you keep the snapshot volumes longer, it is more likely that
more data will be modified. A commonly-used capacity of a secondary
volume is 20% of the source volume.
However, not all write commands would consume space of the
secondary volume. For single block on the primary and snapshot volume,
the copy operation and space allocation on the secondary volume is
performed only at the very first time when a write command hits the
block. As a result, if write commands tend to hit the same blocks, you may
consider using a smaller secondary volume.