If a particular media type is being used in a tape library, IBM suggests setting a scratch threshold value that is greater than zero. This value indicates that the media type is being used in the library. This value also is used for scratch threshold processing, allowing the CBR3660A message to be issued when the number of scratch volumes falls below the scratch threshold setting.

You need to determine whether a library has scratch volumes for the media types requested. A preferred library is one in which each media type requested (or rewritable media inferred from a recording technology specification) for the allocation request has a scratch volume count that is at or above the scratch threshold value. In an ATL, the scratch count is derived from the hardware, so scratch volumes that have been placed in the software error category are not considered usable scratch volumes and are not included in the scratch count. Unless the physical WORM media type is explicitly requested by data class (MEDIA6, MEDIA8, MEDIA10, and MEDIA12), it is excluded from the criteria when determining whether a library is preferred.

The search for a prospective library uses the applicable media type. The applicable media type is based either on the explicit type requested or based on the applicable rewritable media type for the recording technology requested through data class. If the library scratch threshold for the requested (or applicable) media type is zero, the system assumes that the media type is not being used in the library. This library is not considered to be a preferred library. A library with a scratch threshold setting above zero and with the number of scratch volumes at or greater than the threshold value becomes a candidate for the request and is a preferred library. However, a library with a scratch threshold setting above zero and with a scratch count below the threshold value is not considered to be a preferred library.

If you do not use data class to specify a particular media type or recording technology, all rewritable media types are applicable for the scratch request. All media types with a nonzero scratch threshold value are examined to determine if their scratch count is above or below the threshold. If any of the media types in the library are below the threshold, the library is not preferred. If all the media types with a nonzero scratch threshold setting are at or above threshold, then the library is preferred. If the scratch threshold for all the media types are set to zero, OAM cannot determine which media types exist in the library. By default, the library is considered preferred as it is likely that there are scratch volumes for the media type. However, as the example below explains, this situation can lead to a workload imbalance among multiple libraries depending on whether data class is used for the media interchange.

The following is an example where the scratch threshold setting would impact workload balancing if some of the eligible libraries had specified a scratch threshold value and other libraries had not. This scenario uses data class to explicitly request MEDIA2. The following three libraries are eligible for the request.

When applying scratch threshold factors for the MEDIA2 request, the first two libraries (ATLVTS1 and ATLVTS2) are marked below threshold as OAM assumes that MEDIA2 is not being used in the library because the scratch threshold values are set to zero. The third library (ATLVTS3) is a preferred library because the number of scratch volumes is greater than the scratch threshold setting. Until the third library (ATLVTS3) falls below its scratch threshold for MEDIA2, ATLVTS3 with the specified scratch threshold of 1000 will be preferred and scratch requests probably will be allocated to drives in this library, causing a scratch threshold workload imbalance along the three libraries. In this scenario, providing a reasonable or equivalent scratch threshold setting for the first two VTS libraries creates a better balance for MEDIA2 scratch requests.

However, in the above example, if data class is not being used to request a particular media type, each of the three libraries are considered to be above threshold and are preferred, resulting in a better workload balance across the three libraries. With the two VTS libraries (ATLVTS1 and ATLVTS2) not having any scratch threshold values set, by default, these libraries would be considered above threshold.