The storage class accessibility attribute defines the function of the hardware supporting the point-in-time copy, using either concurrent copy, virtual concurrent copy, or FlashCopy®. The point-in-time copy allows database management systems (DBMSs) to take what appears to be an instantaneous copy of data or a "fast" point-in-time copy. The copy can be for backup purposes (generally to tape) or for copying a database from one set of DASD volumes to another. The accessibility attribute allows you to direct allocation of new data sets to DASDs connected to an IBM® 3990 Storage Control unit with cache that supports the concurrent copy function, IBM RAMAC Virtual Array (RVA) devices with the SnapShot copy support, DFSMSdss with the virtual concurrent copy, and the IBM ESS with the FlashCopy service.

The IBM Enterprise Storage Server® (ESS) FlashCopy service provides the appearance of instantaneous replication of a range of track images. You can invoke this service with DFSMSdss and use it to create copies for disaster recoveries, business intelligence applications, data in a test environment, and instantaneous checkpoints.

See z/OS DFSMS Advanced Copy Services for further information on the ESS FlashCopy service.

When you specify accessibility attributes, you identify whether you want SMS to use versioning or backup devices for either concurrent copy or virtual concurrent copy.

Versioning Device

Can create a fast point-in-time version of a data set, which is then available for application testing, reporting, or backup operations. While the version is being made, the data set is unavailable for normal application processing for a minimal period of time. Versioning is done using the SnapShot feature of the RAMAC Virtual Array or the ESS FlashCopy service.

Backup Device

Can create a fast point-in-time backup copy of a data set. While the backup copy is being made, the data set is unavailable for normal application processing for a minimal period of time. Two methods are supported:

Method 1

Establish a concurrent copy session with the 3990 DASD controller and make the backup copy.

Method 2

Use virtual concurrent copy. DFSMSdss uses the SnapShot feature of the RAMAC Virtual Array or the ESS FlashCopy service to create a point-in-time copy and then does I/O to create the backup to whatever target device you specified.

You can use a combination of values for the accessibility attribute and its subparameters, Versioning and Backup, to request the point-in-time copy. In the Accessibility field, you specify whether allocation to a point-in-time copy-capable volume is required (CONTINUOUS), preferred (CONTINUOUS PREFERRED), or discouraged (STANDARD). You then specify values for the Versioning and Backup subparameters to select which devices you want used for the copy.

Table 1 identifies the values you specify based on your specific accessibility requirements:

Accessibility = C
  Versioning  = Y
  Backup      = N

Accessibility = C
  Versioning  = N
  Backup      = Y

Accessibility = C
  Versioning  = blank
  Backup      = blank

Accessibility = P
  Versioning  = blank
  Backup      = blank

Accessibility = P
  Versioning  = Y
  Backup      = Y

Accessibility = P
  Versioning  = Y
  Backup      = N

Accessibility = S
  Versioning  = blank
  Backup      = blank

Accessibility = N
  Versioning  = blank
  Backup      = blank

You can also use the data set alter application to alter a storage class so that data sets can be allocated on point-in-time copy-capable volumes.