Millisecond response time and cache management

At run time, when control unit caching is available, data is divided into three categories: must cache, may cache, and never cache.

Dynamic data set cache management is an SMS feature that permits expanded use of a storage controller that supports dynamic cache management enhancement (DCME) with cache, caching and DASD fast write features when these features are under-used or provides restricted use when they are overused. Only system-managed data sets that reside on volumes attached through a storage controller that supports DCME with cache are affected. Data sets not managed by SMS and data sets that reside on volumes attached through storage controls other than a storage controller that supports DCME with cache are not affected.

Dynamic cache management improves performance when a storage controller that supports DCME with cache is overloaded. It implements a cache management algorithm that optimizes the selection of data sets that are cache candidates. This enhancement also prevents over-commitment of nonvolatile storage. New I/O statistics by data set and SMS storage class can be collected and monitored by using the System Management Facility (SMF). Refer to z/OS MVS System Management Facilities (SMF) for details on the structure of SMF type 42 records.

As a function of dynamic data set cache management, each system-managed data set is assigned a cache usage attribute and a DASD fast write usage attribute for sequential and direct accessing modes. These usage attributes are based on the MSR and bias specifications found in the storage classes associated with the data set, as well as the performance capabilities of the device. For a list of the MSR and bias fields, refer to Defining performance objectives.

Cache usage attribute.
The data set might be assigned one cache usage attribute for sequential accessing mode and a different attribute for direct accessing mode. The possible values that can be assigned for cache usage are:
Must Cache
The respective MSR specification can be met only through the use of cache. BIAS=Read, Write or blank.
As the name implies, this is data that must always be cached in order to meet the performance requirement. If you specify an MSR that is lower than any device can provide without caching, then:
Allocation places the data on a device closest to the requested MSR that has cache active (if one is available) and

The data is cached at execution time (when cache is active) because the MSR cannot be met by the uncached performance of the device.
It is possible that as new devices are introduced, equivalent MSR might be obtainable without the use of cache. In that case, the data might be allocated to a noncached device, but the MSR is still achievable.
May Cache

The respective MSR specification can be met without the use of cache. BIAS=Read, Write or blank.
Any data that is on a cache capable control unit is considered a candidate for may cache. At execution time, if the data is neither must cache nor never cache, dynamic cache management determines if and when to cache the data.

Never Cache

Never Cache data is data that is known to be cache unfriendly. An MSR of 999 indicates that data is never to be cached at execution time, even if it resides on a cache active device. SMS volume selection prefers a volume whose performance is equivalent to 25ms and has Cache inactive. See Table 1.
DASD fast write usage attribute:
The data set might be assigned one DASD fast write usage attribute for sequential accessing mode and a different attribute for direct accessing mode.
Must DASD Fast Write

The respective MSR specification cannot be met without the use of DASD fast write. BIAS=Write.

May DASD Fast Write

The respective MSR specification can be met without the use of DASD fast write. BIAS=Write, Read or blank.

Never DASD Fast Write

The respective MSR specification is 999.

Recommendation: Direct your IDCAMS query regarding the status of cache/DFW to the controller, because querying the device can lead to incorrect conclusions.