content_image/1005870.jpg content_image/1005871.jpg content_image/1005872.jpg
## DFSMShsm-supported data set sizes 

Single-file format, as used by DFSMShsm, reduces I/O and system serialization because only one label is required for each connected set (as opposed to multiple file format tapes that require a label for each data set). The standard-label tape data set that is associated with the connected set can span up to the allocation limit of 255 tapes. This standard-label tape data set is called the  DFSMShsm tape data set . Each user data set is written in 16 K logical blocks to the DFSMShsm tape data set. 

Important:  A single DFSMShsm user data set can span up to 40 tapes (with z/OS V2R1, this limit is now 254). This limit is for migration, backup, and recycle. 

After DFSMShsm writes a user data set to tape, it checks the volume count for the DFSMShsm tape data set. If the volume count is greater than 215, the DFSMShsm tape data set is closed, and the currently mounted tape is marked full and is de-allocated. 

The number 215 is used so that a data set spanning 40 tapes fits within the 255-volume allocation limit. DFSMShsm selects another tape, and then starts a different DFSMShsm tape data set. Data set spanning can be reduced by using the SETSYS TAPESPANSIZE  command. 

## DFSMShsm and large logical volumes 

The TS7700 supports logical volume sizes of 400, 800, 1000, 2000, 4000, 6000, and 25000 MiB. In z/OS V1.13, with a maximum of 40 volumes that are supported and assuming 

a compression ratio of 2.5:1, the maximum user data set size for 800 MiB volumes is 80 GiB: 

<img src='content_image/1005870.jpg'>

In z/OS V2.1 and higher, the limit is 254 volumes for HSM user data sets, so the maximum user data set becomes 508,000 MiB: 

<img src='content_image/1005871.jpg'>

Assume that you have a large data set of 300 GiB. This data set does not fit on 40 volumes of 800 MiB each, but it can fit on 6000 MiB large virtual volumes, as shown in the following example: 

<img src='content_image/1005872.jpg'>

However, in z/OS V2.1 and higher, this data set can fit on 800 MiB volumes. Any single user data set larger than 3.81 TiB at z/OS 1.13 or 15.875 TiB in z/OS 2.1 and higher is a candidate for native 3592 tape drives. Assuming a compression rate of 2.5:1, they might not fit onto the supported number of volumes. In this case, consider the use of native 3592-E06 (TS1130) or 3592-E07 (TS1140) tape drives rather than TS7700. 

IDCAMS DCOLLECT BACKUPDATA  can be used to determine the maximum size of backed-up data sets in DFSMShsm inventory. MIGRATEDATA  can be used to determine the maximum size of migrated data sets in DFSMShsm inventory. 

