The primary volume shadowing operations used to create shadow sets and to maintain consistent data on each member are mount, copy, assisted copy, minicopy (introduced in OpenVMS Version 7.3), merge, and minimerge. When these operations are in progress, the system continues to process read and write requests, thus providing continuous availability.

All volume shadowing operations, except for merges and minimerges, are under the control of the system manager. Merges and minimerges are started automatically by the volume shadowing software if a hardware or software failure occurs that could affect the consistency of the data on the shadow set members. However, you can control the order of these merges by assigning different priorities to the shadow sets, as described in “Prioritizing Merge and Copy Operations”. You can also change the default delays that affect merges and copies with the SHADOW_PSM_DLY and SHADOW_REC_DLY system parameters that are described in Table 3-1.

System managers turn on volume shadowing with the SHADOWING system parameter. They can control the number of concurrent merge or copy operations on a given node by the SHADOW_MAX_COPY system parameter. These volume shadowing system parameters, and all other system parameters used with volume shadowing, are described in “Volume Shadowing Parameters ” and in “Bitmap System Parameters ”.

Volume Shadowing for OpenVMS is never invoked directly. Instead, you invoke the DCL commands MOUNT and DISMOUNT. The MOUNT command works with the volume shadowing software to create shadow sets. The DISMOUNT command works with the volume shadowing software to remove shadow set members and to dissolve entire shadow sets.

HSJ and HSC controllers, when present in a configuration, provide software assists for the minimerge and assisted copy operations. Host-based minimerge (HBMM), introduced in OpenVMS Alpha Version 7.3-2 and in OpenVMS Integrity servers Version 8.2, enables minimerge operations on Fibre Channel and SCSI disk devices.

OpenVMS also provides a programming interface for creating and managing shadow sets with the $MOUNT, $DISMOU, and $GETDVI system services. This programming interface is described in Chapter 5.

Table 1-1 shows the main volume shadowing tasks, the operations associated with them, and the software used to perform the operation. These operations are described in more detail in Chapter 4, Chapter 6, and Chapter 7.