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: 

GLYPH<SM590000> DFSMS striping 

GLYPH<SM590000> Access method compression 

GLYPH<SM590000> 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 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. 

## 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. 

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: 

GLYPH<SM590000> Table scans 

GLYPH<SM590000> Logging 

GLYPH<SM590000> Utilities 

GLYPH<SM590000> Use of DFSMS striping for Db2 data sets 

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. 

