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 with 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.
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
Write 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.
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
Creating and Managing Shadow Sets with System Services.
Table 1 Main Volume Shadowing Tasks, Operation Name, and Related
Software
Task
Operation
Software Used
Create a single-member shadow
set.
Mount
MOUNT/SHADOW command with
the SHADOWING system parameter set.
Create a multiple-member shadow
set
Mount and copy
MOUNT/SHADOW command with
the SHADOWING system parameter set. When a second or third member
is added, the shadowing software starts a copy operation automatically.
Remove a member
from a shadow set.
Dismount a
device
DISMOUNT command.
Dissolve a
shadow set.
Dismount the
shadow set (specify its virtual unit name)
DISMOUNT command.
Ensure 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, 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.