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Appendix D Quality of Service Guide
Callbacks
StorNext 3.5 Installation Guide 155
In the above diagram, a process on client A requests some amount of
RTIO in Step 1. Since this is the first request, the FSM issues callbacks to all
connected clients (Steps 2-5) informing them that the stripe group is now
in real-time mode. The clients respond to the FSM in Steps 6-9. After all
the clients have responded, the FSM responds to the original requesting
client in Step 10.
If the timer expires and one or more clients have not responded, the FSM
must retract the callbacks. It issues a response to the requesting client
with the IP number of the first client that did not respond to the callback.
This allows the requesting client to log the error with the IP number so
system administrators have a chance of diagnosing the failure. It then
sends out callbacks to all the clients to which it first sent the callbacks,
retracting them to the original state. In our example, it would set the
stripe group back to non-real-time mode.
After sending out the callbacks, the FSM waits for a response using the
RtTokenTimeout value as before. If a client again does not respond within
the timeout value, the callbacks are retracted and sent out again. This
repeats until all clients respond. During this time of token retractions,
real-time requests cannot be honored and will only be enqueued.
Callback Failures 4
The FSM must handle a case where a client does not respond to a callback
within the specified timeout period (
RtTokenTimeout). If a client does not
respond to a callback, the FSM must assume the worst: that it is a rogue
that could wreak havoc on real-time I/O. It must retract the tokens it just
issued and return to the previous state.
As mentioned earlier, the original requestor will receive an error
(
EREMOTE) and the IP address of the first client that did not respond to
the callback. The FSM enters the token retraction state, and will not honor
any real-time or token requests until it has received positive
acknowledgement from all clients to which it originally sent the
callbacks.