HP OpenVMS I/O User's Reference Manual |
Shadow-Set Virtual Unit Driver |
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Functions of the Shadow Set Virtual Unit Driver summarizes the major SHDRIVER functions.
| The 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. |
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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