Specifying the desired number of ML2 partial tapes

ML2 tapes are written by the various migration functions and by the recycle of ML2 tapes. A relatively small number of ML2 partially filled tapes are needed as substitutes for tapes that are receiving output when they are taken away by a recall task. The partially filled ML2 tapes that are used as substitutes are not currently associated with specific ML2 output tasks for any hosts.

These substitute tapes are ML2 "floaters." To have the recycle process limit the number of floaters to nnn, specify:

SETSYS ML2PARTIALSNOTASSOCIATEDGOAL(nnn)

To allow an unlimited number, specify:

SETSYS ML2PARTIALSNOTASSOCIATEDGOAL(NOLIMIT)

Floaters are writeable ML2 tapes that are not assigned or associated with a specific task in a specific host. When a floater substitutes for a tape that is taken away, it becomes an associated tape, restricted for output to that one specific task.

An output tape taken away by recall is not immediately available to receive output again. It becomes available for output on the host where the recall occurred as the recall completes. A long queue of recalls from that tape would likely cause a delay in considering such a tape for output again.

DFSMShsm hosts other than the one doing the recall do not become aware of a tape's availability for output until their own in-storage selection table is depleted and rebuilt. For performance reasons, in-storage selection tables are not rebuilt more often than every 45 minutes.

A configuration where a storage group uses multiple SMS tape libraries can affect the number of floaters that are needed. As functions such as primary space management, interval migration, or recycle start, they can allocate a drive from any of the libraries associated with the storage group. A tape must be in the same library as the drive that is allocated. This tends to increase the number of floaters needed, even without a high tape take-away rate. The tape associated with the previous processing of a task may not be part of the library that contains the allocated drive. In this case, a floater is used, and the previously associated tape is placed in its host's TVT.

From the preceding descriptions, you can see how data usage and configuration can result in a need for a higher number of ML2 floaters. Having floaters is a trade-off between having a few additional under-utilized ML2 tapes, or recycling the additional few under-utilized ML2 tapes. The DFSMShsm design is to extend a partially filled ML2 tape instead of using an empty tape. However, in cases where partially filled ML2 tapes do not exist, DFSMShsm requests a scratch tape.

It is only when recycle of ML2s occurs that the value specified by ML2PARTIALSNOTASSOCIATEDGOAL is used. The default used for ML2PARTIALSNOTASSOCIATEDGOAL is 10. This value applies to the total number of ML2 not-associated partial tapes in an entire HSMplex, but only for tapes that meet the recycle selection criteria of PERCENTVALID and any SELECT filters. If the number of such floaters, at the time a recycle command runs, exceeds the specified value, then the excess tapes are included as candidates for recycle. The floaters that are thus recycled are those having the lowest percentage of valid data.

You can use the LIST TTOC SELECT(NOTASSOCIATED) command to get a list of all not-associated ML2s if you want to know how many or which ones they are. If you observe a large number of not-associated ML2 partials being recycled, and this seems to occur consistently, then you probably want to increase the SETSYS specification to reduce the recycle load. This is just a trade-off between having the few under-utilized tapes or recycling the tapes so you do not have them (at least for a while).

You can use the LIST TTOC SELECT(ASSOCIATED) command to get a list of all ML2 partials that are associated with specific ML2 output tasks. If some of these are associated with tasks that are unused, or seldom used, then you may want to specify DELVOL UNASSIGN so that they will be part of the set of floaters. To find out which ML2 tapes are associated with what tasks, display the various host MCR records using the FIXCDS command. If they are old-technology cartridges, or recorded in old technology not being written anymore, you should use the DELVOL MARKFULL command so they can be recycled when they meet that criteria.

One approach for determining a unique setting for your installation is to mark the existing ML2 partial tapes as full. Then set ML2PARTIALSNOTASSOCIATEDGOAL to NOLIMIT for a while, and issue the LIST TTOC command with SELECT(ASSOCIATED) and SELECT(NOTASSOCIATED) every few days. When the sum of these two categories settles down or reaches a maximum, your migration workload indicates that you can set ML2PARTIALSNOTASSOCIATEDGOAL to or slightly below the number of not-associated partials indicated by the LIST TTOC command.