Figure A.5 State transition diagram
Session operation | Express function |
|---|---|
Start | acec start |
Suspend | acec suspend |
Resume | acec resume |
Reverse | acec reverse |
Stop | acec cancel |
Start
When the EC session is started, the whole area of data specified by the source parameters is copied.
If data copying has been completed for a given area and a write operation is applied to that area, the write data is also transferred to the destination area.
After all data in the specified area has been copied over, the data in source and destination areas will be kept in almost the same state by the EC function. This is called the Equivalent state. In the Equivalent state, if source data is changed, the same data is transferred to the destination area to maintain the Equivalent state.
Until the EC session reaches to the Equivalent state, copy destination volume are neither readable nor writable. If EC session is in the Equivalent state, copy destination volume are not writable but readable.
Suspend
When the EC session is suspended, both the copy source volume and the copy destination volume are accessible as independent volumes (copy destination volume becomes readable and writable). The Suspend operation can only be executed when the EC session is in the Equivalent state.
Unlike the Stop operation, changes to the source and destination volumes continue to be recorded while the Suspend state applies.
Resume
When the EC session is resumed, data is copied over from the source to the destination so as to catch up on all the changes that were made after the Suspend operation was performed and recover the Equivalent state. As the amount of data that thus needs to be transferred is much less than would be required if the whole source area were to be recopied, the time required to recover the Equivalent state is also greatly reduced.
If the source is changed under the Suspend state (when copying is temporarily interrupted by the Suspend operation), the changes are sent to the destination after the Resume operation is performed (when the copy is restarted). However, any changes made at the destination while under the Suspend state will be rolled-back (overwritten from the source data). Consequently, any and all changes made to the destination during the Suspend state are discarded and lost following the Resume operation.
Concurrent Suspend
Multiple EC sessions can be suspended using the Concurrent Suspend operation. This allows consistent copies of multi-volume data-objects, such as databases, to be easily acquired.
Reverse
When the EC session is reversed, the copy direction is reversed. The Reverse operation can only be executed when the EC session data is in the Suspend state.
Stop
When the EC session is stopped, it terminates. If the EC session in one of the following statuses is stopped, the copy destination volume should not be used because the mirroring is incomplete.
Initial copy state
Error suspend state
If the EC session in one of the following statuses is stopped, the copy destination volume can be used because the mirroring has been complete.
Equivalent state
Suspend state
