DFSMShsm
The z/OS storage manager that moves datasets you have not used for a while to cheaper storage, and brings them back when you need them.
Also written HSM, DFHSM, MIGRAT, HRECALL, recall, migrated dataset
DFSMShsm, usually called HSM, manages the life of datasets after they are created. Its best-known job is migration: moving datasets that have not been used for a while off the main disks onto cheaper storage, to free up space.
There are two migration levels. Level 1 is compressed copies on disk. Level 2 is usually tape, or virtual tape. In ISPF 3.4 a migrated dataset shows MIGRAT, MIGRAT1 or MIGRAT2 where the volume name would normally be. The dataset still exists and is still catalogued. It is just stored somewhere else.
When you open a migrated dataset, HSM recalls it automatically, which can take a few seconds from disk or several minutes from tape. You can also ask for it in advance with the HRECALL command, which you can also type next to the dataset in ISPF 3.4.
HSM also takes backups of datasets and can restore them, with commands such as HBACKDS and HRECOVER. How long a dataset waits before migrating, and how many backups are kept, comes from its SMS management class.
Where can I learn this hands-on?
Right here, at ZedBank. You take a junior job at a simulated bank and learn each part of the work by doing it, with Zed beside you. There's nothing to install.
Start at ZedBankRelated
- SMSThe Storage Management Subsystem: the z/OS feature that decides where new datasets go and how they are backed up, kept and deleted, using rules the site writes.
- DASDDirect access storage device, the mainframe term for disk storage.
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- CatalogThe system's index of where datasets live, so you can refer to a dataset by name without knowing which disk it is on.
- DatasetWhat a mainframe calls a file: a named collection of records with a defined structure, rather than a loose stream of bytes.
- DSLISTISPF option 3.4, the screen for listing datasets by name and working with them from the list.