z/OS operating system support of EAV

There are several things to consider before deciding to put datasets on EAV, and deciding between CMS and TMS.

Datasets on EAV

z/OS support for EAV was introduced in release 1.10 (DS8000 Release 4.0 Licensed Internal Code is also required). A DS8700 can define CKD volumes with up to 262,668 cylinders (~223 GB) and a DS8800 can define 1,182,006 cylinders (~1 TB) per volume. An EAV is a 3390A with more than 65520 cylinders. It is divided into the base addressing space (BAS) containing cylinders 0-65535 and the extended address space (EAS) containing the rest. An EAV is also divided into the track managed space (TMS) containing cylinders 0-65519, and cylinder managed space (CMS) containing the rest.

EATTR provides a way to directly set the allocation behavior for an EAS eligible data set. This gives the user a way to override the default allocation behavior for any data set, regardless of whether it’s EAS eligible at that OS level. EATTR=NO means it cannot have extended attributes, and so if allocated on an EAV, it is always allocated in the TMS (track managed space). EATTR=OPT means it can have extended attributes, and so if allocated on an EAV, it is biased towards being allocated in the CMS (cylinder managed space). Not specifying EATTR means to use the default allocation behavior for the data set type. For VSAM data sets, the default behavior is the equivalent of EATTR=OPT. For non-VSAM data sets, the default behavior is the equivalent of EATTR=NO. The EATTR setting is saved in DS1EATTR in the F1 or F8 DSCB (data set control block).

CMS and TMS

The system uses this logic to decide between CMS and TMS:

Figure 1. EAV Logic. Logic needed to decide between CMS and TMS.

Is the EAS an eligible data set type? If not, the system uses TMS. If it is an eligible data set type, and if the EATTR equals NO, then it uses the TMS. If the EATTR does not equal NO, the system looks at whether the EATTR equals OPT. If the EATTR does not equal OPT, then the system looks at whether the DAB equals OPT. If the DAB does not equal OPT, then it uses TMS. If the DAB does equal OPT, then it considers whether the space request is greater than or equal to the breakpoint value (BPV). If the EATTR does equal OPT, then it considers whether the space request is greater than or equal to the BPV. If the space request is not greater than or equal to the BPV, then it uses TMS. If the space request is greater than or equal to the BPV, the system considers whether there is enough space available. If there is enough space available, it uses the CMS, but if there is not enough space available, it uses the TMS. If the EATTR does not equal OPT, then consider whether the DAB equals OPT. The diagram visually represents the data flow logic that is used to decide between CMS and TMS.

Note: For more information about defining the Break Point Value (BPV), see page 314 in the IBM Redbook DFSMS V1.10 and EAV Technical Guide