7.3.1 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, which will likely return the data the
fastest. See the Volume Shadowing for OpenVMS manual for more information about controlling this behavior.
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:
All members might contain
the new data.
All members might contain
the old data.
Some members might contain
new data and others might contain old data.
When the system recovers, volume shadowing performs
a merge 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:
Permanent loss of the
path between a CPU data buffer containing the data being written and
the disk being written to during a multiple block I/O operation. Communication
path loss can occur due to node failure or a failure of node-to-node
Failure of a CPU (such
as a system crash, halt, power failure, or system shutdown) during
a multiple block write I/O operation.
Mistaken deletion of a
Corruption of file system
OpenVMS RMS file corruption
due to a software error or incomplete bucket write operation to an
Cancellation of an in-progress
multiple block write operation.
For more information about shadowing merge operations,
see the Volume Shadowing for OpenVMS manual.