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Driver Functions  



This section describes the major virtual unit functions supported by SHDRIVER. In addition to the virtual unit functions described in this section, SHDRIVER supports all OpenVMS disk functions. SHDRIVER receives QIO operations from application programs and is a client of the disk class drivers DUDRIVER. Applications access the shadow set as they would access a standard OpenVMS disk.

Functions of the Shadow Set Virtual Unit Driver summarizes the major SHDRIVER functions.


NoteThe MOUNTSHAD, ADDSHADMBR, COPYSHAD, and REMSHADMBR functions are reserved for HP internal use. Use of these functions by customer or third-party provided software may cause unpredictable results including system crashes and data corruption.

Table 1   Functions of the Shadow Set Virtual Unit Driver
Function Description
MOUNTSHAD
Creates a virtual unit
ADDSHAD
Evaluates a physical member and adds members
COPYSHAD
Triggers and controls copy operations
REMSHAD
Removes a physical member
AVAILABLE
Virtual unit dissolution
SENSECHAR
Verifies shadow set status
READ
Directs I/O to a physical member
WRITE
Propagates a write operation to all physical members

Read and Write Functions  

With minor changes, the read and write functions for SHDRIVER operate the same as for the disk class driver (see Read and Write).

During an SHDRIVER read operation, the host directs the read to the member volume with the shortest path.

During a write operation, SHDRIVER directs the write to each member volume. The write operations for each member volume usually proceed in parallel; the virtual unit write operation terminates when all writes have completed. The write function for SHDRIVER takes the IO$M_VUEX_FC function modifier; this modifier should not be used by application programs.

The read and write SHDRIVER functions, as well as all user functions, are issued by user programs. All other SHDRIVER functions are invoked by MOUNT and DISMOUNT commands, or the $MOUNT and $DISMOUNT system services.

Remember that volume shadowing provides data availability by protecting against hardware problems or communication path problems that might cause a disk volume to be a single point of failure. If a write request is made to a shadow set, but the system fails before a completion status is returned from all of the shadow set members, it is possible that:

When the system recovers, volume shadowing performs a merge or copy operation to ensure that the corresponding blocks on each shadow set member contain the same data (right or wrong); therefore, the goal here is not one of data correctness but of data availability. Volume shadowing is designed to make the data on all disks identical, then, if necessary, incorrect data can be reconciled either by the user reentering the data or by an application automatically employing database or journaling techniques.

For example, when used with volume shadowing, OpenVMS RMS journaling allows you to develop applications that can automatically recover from failures such as:

Refer to the Volume Shadowing for OpenVMS manual for more information about shadowing merge and copy operations.


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