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
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
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.
Table 1-1 Main Volume Shadowing Tasks, Operation Name, and Related Software
| Task|| Operation|| Software
a single-member shadow set.
MOUNT/SHADOW command with the SHADOWING system parameter set.
a multiple-member shadow set
Mount and copy
MOUNT/SHADOW command with the SHADOWING system parameter
When a second or third member
is added, the shadowing software starts a copy operation automatically.
In OpenVMS Version 8.4, you can add up to six members.
a member from a shadow set.
Dismount a device
a shadow set.
Dismount the shadow set (specify its virtual unit name)
that the data is identical on all shadow set members in the event
of a hardware failure.
Merge or minimerge
Shadowing software does this automatically when it
detects a hardware or software failure. If an HSJ or HSC controller
is present in the configuration, or your system and shadow sets are
configured for HBMM, a minimerge might be done.
Return a dismounted
shadow set member to the shadow set.
Copy, assisted copy, or minicopy
MOUNT command, with shadowing software that initiates
either a copy or, if properly configured, a minicopy.