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Performance During Steady State  



A shadow set is in a steady state when all of its members are consistent and no copy operation or merge operation is in progress. Overall, the performance of a shadow set in a steady state compares favorably with that of a nonshadowed disk. Read and write I/O requests processed by a shadow set utilize a very small amount of extra CPU processing time as compared with a nonshadowed disk. A shadow set often can process read requests more efficiently than can a nonshadowed disk because it can use the additional devices to respond to multiple read requests simultaneously.

For a shadow set in a steady state, the shadowing software handles read and write operations in the following manner:

Even though the read performance of a shadow set in steady-state has the potential for better performance, the primary purpose of volume shadowing is to provide data availability. It is not appropriate to use volume shadowing as a means to increase the read I/O throughput of your applications (by explicitly increasing the I/O work load). This is because the same level of performance cannot be expected during situations when copy or merge operations must take place to add new members or preserve data consistency, or when members are removed from the shadow set. Performance During Copy and Merge Operations discusses performance considerations when the shadow set is in a transient state.


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