## 4.4.2 Virtual volumes 

Determine the number of virtual volumes that are required to handle the workload that you are planning for the TS7700. The default number of logical volumes that is supported is 1,000,000. You can add support for more logical volumes in 200,000 volume increments (FC5270), up to a total of 4,000,000 logical volumes. 

Tip:  For 3957-V06/VEA, the maximum is 2,000,000 virtual volumes, which also is the limit for the overall grid that contains clusters that correspond to one of those models. 

The TS7700 supports logical WORM (LWORM) volumes. Consider the size of your logical volumes, the number of scratch volumes you need per day, the time that is required for return-to-scratch processing, how often scratch processing is run, and whether you need to define LWORM volumes. 

## Size of virtual volumes 

The TS7700 supports logical volumes with maximum sizes of 400, 800, 1000, 2000, 4000, 6000, and 25,000 mebibytes (MiB), although effective sizes can be larger if data is compressed. For example, if your data compresses with a 3:1 ratio, the effective maximum logical volume size for a 6000 MiB logical volume is 18,000 MiB. 

Depending on the virtual volume sizes that you choose, you might see the number of volumes that are required to store your data grow or shrink, depending on the media size from which you are converting. If you have data sets that fill native 3590 volumes, even with 6000 MiB virtual volumes, you need more TS7700 virtual volumes to store the data, which is stored as multivolume data sets. 

The 400 MiB cartridge storage tape (CST)-emulated cartridges or 800 MiB with emulated enhanced capacity cartridge system tape (ECCST)-emulated cartridges are the two types you can specify when adding volumes to the TS7700. You can use these sizes directly, or use policy management to override them to provide for the 1000, 2000, 4000, 6000, or 25,000 MiB sizes. 

A virtual volume size can be set by VOLSER, and can change dynamically by using the DFSMS DC storage construct when a job requires a scratch volume or writes from the beginning of tape (BOT). The amount of data that is copied to the stacked cartridge is only the amount of data that was written to a logical volume. The choice between all available virtual volume sizes does not affect the real space that is used in the TS7700 cache or the stacked volume. 

In general, unless you have a special need for CST emulation (400 MiB), specify the ECCST media type when you insert volumes in the TS7700. 

In planning for the number of logical volumes that is needed, first determine the number of private volumes that make up the current workload that you are migrating. One way to determine this number is by looking at the amount of data on your current volumes and then matching that figure to the supported logical volume sizes. Match the volume sizes, accounting for the compressibility of your data. If you do not know the average ratio, use the conservative value of 2:1. 

If you choose to use only the 800 MiB volume size, the total number that is needed might increase depending on whether current volumes that contain more than 800 MiB compressed need to expand to a multivolume set. Consider that fact when planning the number of logical volumes required. Consider the use of smaller volumes for applications, such as DFSMShsm and larger volumes for backup and full volume memory dumps. 

