Extended Format Data Sets - IBM z13s Technical Manual

when the CCW count goes to zero or the last MIDAW (with the last flag) ends. The
combination of the address and count in a MIDAW cannot cross a page boundary. This
configuration means that the largest possible count is 4 K. The maximum data count of all the
MIDAWs in a list cannot exceed 64 K, which is the maximum count of the associated CCW.
The scatter-read or scatter-write effect of the MIDAWs makes it possible to efficiently send
small control blocks that are embedded in a disk record to separate buffers from those used
for larger data areas within the record. MIDAW operations are on a single I/O block, in the
manner of data chaining. Do not confuse this operation with CCW command chaining.

7.11.2 Extended format data sets

z/OS extended format (EF) data sets use internal structures (usually not visible to the
application program) that require a scatter-read (or scatter-write) operation. Therefore, CCW
data chaining is required, which produces less than optimal I/O performance. Because the
most significant performance benefit of MIDAWs is achieved with EF data sets, a brief review
of the EF data sets is included here.
Both VSAM and non-VSAM (DSORG=PS) sets can be defined as extended format data sets.
For non-VSAM data sets, a 32-byte suffix is appended to the end of every physical record
(that is, block) on disk. VSAM appends the suffix to the end of every control interval (CI),
which normally corresponds to a physical record. A 32 K CI is split into two records to span
tracks. This suffix is used to improve data reliability and facilitates other functions that are
described in the following paragraphs. Therefore, for example, if the DCB BLKSIZE or VSAM
CI size is equal to 8192, the actual block on storage consists of 8224 bytes. The control unit
itself does not distinguish between suffixes and user data. The suffix is transparent to the
access method and database.
In addition to reliability, EF data sets enable three other functions:
DFSMS striping
Access method compression
Extended addressability (EA)
EA is useful for creating large DB2 partitions (larger than 4 GB). Striping can be used to
increase sequential throughput, or to spread random I/Os across multiple logical volumes.
DFSMS striping is useful for using multiple channels in parallel for one data set. The DB2 logs
are often striped to optimize the performance of DB2 sequential inserts.
Processing an I/O operation to an EF data set normally requires at least two CCWs with data
chaining. One CCW is used for the 32-byte suffix of the EF data set. With MIDAW, the
additional CCW for the EF data set suffix is eliminated.
MIDAWs benefit both EF and non-EF data sets. For example, to read twelve 4 K records from
a non-EF data set on a 3390 track, Media Manager chains 12 CCWs together by using data
chaining. To read twelve 4 K records from an EF data set, 24 CCWs are chained (two CCWs
per 4 K record). Using Media Manager track-level command operations and MIDAWs, an
entire track can be transferred by using a single CCW.
7.11.3 Performance benefits
z/OS Media Manager has I/O channel program support for implementing EF data sets, and
automatically uses MIDAWs when appropriate. Most disk I/Os in the system are generated by
using Media Manager.
Chapter 7. Software support
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