Performance Implications of Bitmaps

There are several aspects of bitmaps that affect performance; the message traffic that occurs between local and master bitmaps, the size requirements of each bitmap, asynchronous processing of SetBit messages, and reduced SetBit messages for sequential I\O.

The message traffic can be adjusted by changing the message mode. Single message mode is the default mode. Buffered message mode can improve the overall system performance, but the time to record the write of each process in the master bitmap usually takes longer. These modes are described in detail in “Bitmap System Parameters ”.

	NOTE: Additional memory is required to support bitmaps, as described in “Memory Requirements”. Depending on the memory usage of your system, it may require additional memory.

There can be multiple master bitmap nodes for a shadow set. In OpenVMS Version 8.3 and earlier, SetBit messages are sent to the multiple master bitmap nodes synchronously. Only when the response for the SetBit message is received from the first remote master bitmap node, is the message sent to the next master bitmap node. When this process completes for all the remote master bitmap nodes, the I/O resumes.

In OpenVMS Version 8.4, SetBit messages are sent to all multiple master bitmap nodes asynchronously. I/O resumes when the responses from all the master bitmap nodes are received, thus reducing I/O delay by the write bitmap code.

In earlier versions, if sequential writes are occurring to a disk, these writes often resulted in delivering Setbit messages that set sequential bits in the remote bitmap. In OpenVMS Version 8.4, the write bitmap code recognizes where a number of prior bits in the bitmap are set. In this case, additional bits are set so that if sequential writes should continue, fewer Setbit messages are required. Assuming that the sequential I/O continues, the number of Setbit messages are reduced by about a factor of 10, thus improving the I/O rate for sequential writes.