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More on Snapshot
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Appendix C
More on Snapshot
C.1 Overview
This appendix is provided to give a more in depth look at Snapshot, in order to provide a better
understanding of the technology used.
The Snapshot engine has been designed to give the user considerable flexibility when setting up and
performing snapshot and snapback operations. A snapshot allows the user to recreate a point in time
image of a logical drive that contains exactly the same data at the point the it was taken.
To recreate the data of the snapshot, a snapback is initiated. This will cause the controller to copy data
from the Overwrite Data Area (ODA) back to the snapshotted logical drive. Once completed, the logical
drive will now contain exactly the same data as it did at the time of the snapshot.
A snapshot image can also be presented back to the host server as a snapshot (virtual) logical drive, in
which the server will see this virtual LUN and its data just like any other logical drive. However, there is
one exception to snapshot (virtual) logical drives, it is only possible to read from the snapshot logical
drive. Any write commands issued to it will result in a error or check condition.
One of the most important components of Snapshot is the Overwrite Data Area or ODA. The ODA serves
as a storage area for disk data that needs to be protected from being overwritten by new data after a
snapshot was issued.
C.1.1 Terminology
The following describes some of the terminology used when working with Snapshot.
Term
Description
Internal Logical Drive
An internal logical drive is identical to a regular logical drive except that
it is NOT made visible to a host adapter as a LUN. Instead internal
logical drives are used for setting up snapshot ODA that are only
accessed internally by the RAID controller.
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