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Differentiate between Read and Write IO performance (Overhead and TransferRate) and optimise accordingly
Background: Db2 is sensitive to IO performance, and compiles access plans accordingly to optimise IO versus CPU costs. At the tablespace level, average Overhead and TransferRate are configured in units of milliseconds.
Issue: customers such as ourselves have very different Read and Write IO performance. In our case, the underlying disks are mirrored using MetroMirror (synchronous PPRC) which means that reads occur at normal speeds but writes take much longer.
Db2 should be made aware of both Read and Write performance, and should optimize access plans (and other internal functions) accordingly to minimise total CPU, read and write costs.
Note: in Db2 9.7, columns WRITEOVERHEAD and WRITETRANSFERRATE were added to SYSCAT.TABLESPACES, temptingly making it look like Db2 was about to introduce the requested functionality; however these columns are still flagged "Reserved for future use."
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