Figure A.7 State transition diagram
Session operation | Express function |
|---|---|
Start | acopc start |
Differential copy start | acopc start -diff |
Stop | acopc cancel |
Start
When the QuickOPC session is started, the logical copy completes immediately and returns a "copied" response before any actual (physical) copying occurs. The actual physical copy process and tracking process start after the logical copy has finished (the tracking process keeps track of data which is modified for copy source and copy destination volumes after the logical copy). These two processes are internal ETERNUS Disk storage system background processes that are not visible to the server. After the logical copy has completed, the copy destination area becomes accessible. Even if the physical copy has finished, the session remains because the tracking process is still active.
Point
The QuickOPC function allows hardware to record updates that occur on copy source/destination after logical copy has been finished. The state that hardware is recording the updated point is referred to as "Tracking state".
Differential copy start
When the QuickOPC session is restarted, then instead of the initial full copy, only the data areas recorded by the tracking process are copied over from the source to the destination.
While less data needs to be copied, this copy is similar to the initial copy in that a logical copy step precedes the actual physical copy, and the copy destination becomes read/write accessible as soon as the logical copy has completed. The tracking process also starts recording subsequent updates again at this stage.
If the source is changed after the previous logical copy, the changes are sent to the destination after the Restart operation is performed (when the copy is restarted). However, any changes made at the destination after the previous logical copy will be rolled-back (overwritten from the source data). Consequently, any and all changes made to the destination after the previous logical copy are discarded and lost following the Restart operation.
Stop
When the QuickOPC session is stopped, it terminates. If the QuickOPC session which is in the Physical copy and Tracking state is stopped, the copy destination volume cannot be used because the physical copy is incomplete. If the QuickOPC session in the Tracking state is stopped, the copy destination volume can be used because the physical copy has completed.
