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 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 12 CCWs together 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 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. 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. The performance of a specific workload can vary based on the conditions and hardware  configuration of the environment. IBM laboratory tests found that Db2 gains significant  performance benefits by using the MIDAW facility in the following areas: Media Manager with the MIDAW facility can provide significant performance benefits when  used in combination applications that use EF data sets (such as Db2) or long chains of small  blocks. Performance benefits GLYPH<SM590000> DFSMS striping GLYPH<SM590000> Access method compression GLYPH<SM590000> Extended addressability (EA) GLYPH<SM590000> Table scans GLYPH<SM590000> Logging GLYPH<SM590000> Utilities GLYPH<SM590000> Use of DFSMS striping for Db2 data sets Chapter 7. Operating system support  285