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Performance Improvements

The following items improve performance:

  • Fast CCW Translation and Minidisk Caching for 64-bit DASD I/O
  • Block Paging Improvement

Fast CCW Translation and Minidisk Caching for 64-bit DASD I/O

The CCW translation fast path and minidisk caching have now been extended to include 64-bit DASD I/O, resulting in reduced processing time and I/Os. Which cases are eligible and the degree of improvement are equivalent to what is already experienced with 31-bit I/O. (Exception: FBA devices do have fast CCW translation for 64-bit I/O but not MDC support.) See 64-bit Fast CCW Translation for measurement results.

Note: Fast CCW translation for both 31-bit and 64-bit channel-to-channel I/O was provided in z/VM 4.1.0. See Fast CCW Translation for Network I/O for additional information.

Block Paging Improvement

For improved efficiency, CP typically writes pages to DASD in blocks consisting of a number of pages that have all been referenced during the same time period. Later, when a page fault occurs for any of those pages, the entire block is read and all of its pages are made valid under the assumption that the pages in the block tend to be used together.

In prior releases, all pages in a given block were required to reside in the same megabyte of virtual storage. With z/VM 4.2.0, this restriction has been removed. As a result, the set of pages that now make up a block will tend to be more closely related in terms of their reference pattern, resulting in faster average page resolution times and a reduction in overall paging.

The amount of improvement is expected to be small for most environments. The most noticeable improvements are expected for environments that have high DASD page rates and DAT ON guests.


A new LZCOMPACT option can now be specified on the output I/O definition control statement when using DDR to dump to tape. This provides an alternative to the compression algorithm used by the existing COMPACT option. Unlike the COMPACT option, the data compression done when the LZCOMPACT option is specified can make use of the hardware compression facility to greatly speed up data compression (DUMP) and decompression (RESTORE).

Measurement results indicate that use of the LZCOMPACT option will tend to result in the following performance characteristics relative to the COMPACT option:

  • Increased processor time and slightly increased elapsed time on processors that do not have the hardware compression facility.

  • Reduced processor time and slightly decreased elapsed time on processors having the hardware compression facility.

  • A higher compression ratio.

  • The same number of tape I/Os.

  • A decrease in tape length requirements. Decreases ranging from 1% to 28% have been observed for 9 example DASD volumes. This improvement can result in fewer tape cartridges being required, depending on tape cartridge capacity and the amount of data being dumped.

For measurement results and further discussion, see DDR LZCOMPACT Option.

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