Defining bias

Bias determines which volumes MSR performance numbers (read, write, or both) to consider during volume selection. If you specify a read (R) bias, a cache storage control should be available to allow caching. If you specify a write (W) bias, the DASD fast write feature of an IBM® 3990 Storage Controller with cache should be available to allow the use of DASD fast write. If you do not specify a value for bias (blank), the MSR time determines whether caching or DASD fast write are used.

Once the data sets have been allocated, the MSR time determines whether caching or DASD fast write are used. You can inhibit caching or DASD fast write by specifying a MSR time of 999. If a bias is specified without an accompanying MSR, an MSR of 6ms for write bias or an MSR of 10ms for read bias is used for volume selection. If the MSR is blank, then the data set is May Cache and May DASD Fast Write.

Tip: Allocation does not fail when a storage control device does not have caching or DASD fast write available.

Table 1 can help you determine which setting (May, Must, Never) is set by DCME:

Table 1. D/T3990 SMS Cache Candidate Tokens for Sequential and Direct Requests
Direct/Sequential MSR	Dir/Seq Bias	Direct Cache Token		Sequential Cache Token
Direct/Sequential MSR	Dir/Seq Bias	Read	Write³	Read	Write³
999	any	Never	Never	Never	Never
blank	blank	May	May²	May¹	May
MSR < UCr⁴ and MSR < UCw⁵	blank	Must	May²	Must	May
MSR < UCr and MSR >= UCw	blank	Must	May²	Must	May
MSR >= UCr and MSR < UCw	blank	May	May²	May¹	May
MSR >= UCr and MSR >= UCw	blank	May	May²	May¹	May
blank	Read	May	May²	May¹	Never
MSR < UCr and MSR < UCw	Read	Must	May²	Must	Never
MSR < UCr and MSR >= UCw	Read	Must	May²	Must	Never
MSR >= UCr and MSR < UCw	Read	May	May²	May¹	Never
MSR >= UCr and MSR >= UCw	Read	May	May²	May¹	Never
blank	Write	May	May²	May¹	May
MSR < UCr⁴ and MSR < UCw⁵	Write	Must	Must	Must	Must
MSR < UCr and MSR >= UCw	Write	Must	May²	Must	May
MSR >= UCr and MSR < UCw	Write	May	Must	May¹	Must
MSR >= UCr and MSR >= UCw	Write	May	May²	May¹	May
Note: The 3990 Extended Platform results in any sequential read request to always be cached unless MSR 999 is specified. The 3990 Extended Platform results in all direct write requests from VSAM or Media Manager to be treated as must write. All write requests to a RAMAC device are treated as must write. Uncached values read performance. Uncached values write performance.

See Table 1 about performance characteristics of a particular IBM device.

Set the storage class to indicate the desired performance for a data set. SMS volume selection and DCME work together to provide the closest performance possible at the time of allocation and open.