IBM z13s, Technical Manual

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Chapter 7. Software support
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The MIDAW facility provides a more efficient CCW/IDAW structure for certain categories of
data-chaining I/O operations:
MIDAW can improve FICON performance for extended format data sets. Non-extended data
sets can also benefit from MIDAW.
MIDAW can improve channel utilization, and can improve I/O response time. It reduces
FICON channel connect time, director ports, and control unit processor usage.
IBM laboratory tests indicate that applications that use EF data sets, such as DB2, or long
chains of small blocks can gain significant performance benefits by using the MIDAW facility.
MIDAW is supported on FICON channels that are configured as CHPID type FC.
7.11.1 MIDAW technical description
An IDAW is used to specify data addresses for I/O operations in a virtual environment.
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The
existing IDAW design allows the first IDAW in a list to point to any address within a page.
Subsequent IDAWs in the same list must point to the first byte in a page. Also, IDAWs (except
the first and last IDAW) in a list must deal with complete 2 K or 4 K units of data.
Figure 7-3 shows a single CCW that controls the transfer of data that spans non-contiguous
4 K frames in main storage. When the IDAW flag is set, the data address in the CCW points to
a list of words (IDAWs). Each IDAW contains an address that designates a data area within
real storage.
Figure 7-3 IDAW usage
The number of required IDAWs for a CCW is determined by these factors:
򐂰 The IDAW format as specified in the operation request block (ORB)
򐂰 The count field of the CCW
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There are exceptions to this statement, and many details are skipped in this description. This section assumes that
you can merge this brief description with an existing understanding of I/O operations in a virtual memory
environment.