The MSR serves two purposes in SMS. First, it is used as the performance objective for selecting candidate volumes for new data set placement. During a new data set allocation, SMS searches for a volume that meets or closely matches this objective. If no volume satisfies the objective, then SMS attempts to find a volume that comes closest to matching it. If more than one MSR is explicitly or implicitly specified, the storage class and associated device MSRs are averaged and compared.

Second, if the data is placed on a volume attached through an IBM® 3990 Storage Control with cache, and cache is enabled for that volume, the MSR is used to determine if caching is mandatory, optional, or should be inhibited for the data set. This attribute does not apply to objects.

You can request SMS to ignore various device performances during volume selection by leaving all MSR and BIAS fields blank. This lets you spread data evenly across noncached and cache active devices.