Extended format data sets Performance benefits z/OS extended format (EF) data sets use internal structures (often 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. VSAM and non-VSAM (DSORG=PS) sets can be defined as EF 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 next. 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 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 the following functions:  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 EF and non-EF data sets. For example, to read 12 4 K records from a  non-EF data set on a 3390 track, Media Manager chains together 12 CCWs by using data  chaining. To read 12 4 K records from an EF data set, 24 CCWs are chained (two CCWs per  4 K record). By using Media Manager track-level command operations and MIDAWs, an  entire track can be transferred by using a single CCW. z/OS Media Manager includes 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. Users of the Executing Fixed Channel Programs in Real Storage (EXCPVR) instruction can  construct channel programs that contain MIDAWs. However, doing so requires that they  construct an IOBE with the IOBEMIDA bit set. Users of the EXCP instruction cannot construct  channel programs that contain MIDAWs. The MIDAW facility removes the 4 K boundary restrictions of IDAWs and for EF data sets,  reduces the number of CCWs. Decreasing the number of CCWs helps to reduce the FICON  channel processor utilization. Media Manager and MIDAWs do not cause the bits to move any  faster across the FICON link. However, they reduce the number of frames and sequences that  flow across the link, and therefore use the channel resources more efficiently. Chapter 7. Operating system support  299 GLYPH<SM590000> DFSMS striping GLYPH<SM590000> Access method compression GLYPH<SM590000> Extended addressability (EA)