On Wednesday, 19 February 2014 at 05:53:55 UTC, Tolga Cakiroglu
wrote:
On Wednesday, 19 February 2014 at 01:09:43 UTC, Xinok wrote:
On Wednesday, 19 February 2014 at 00:16:03 UTC, Tolga
Cakiroglu wrote:
TL;DR the link though, how are they detecting that a CPU
fails? An information must be passes outside of CPU to do
this. The only solution comes to my mind is that main CPU
changes a variable on an external memory at every step, and
back up CPU checks it continuously to catch a failure
immediately. But this would require about 50% of CPU's power
already.
While thinking about this kind of back up systems, knowing
and reading that some people are really doing is really great.
I'm assuming this has something to do with it:
https://en.wikipedia.org/wiki/Heartbeat_%28computing%29
In clustered servers, the active node sends a continuous
signal indicating it's still alive. This signal is referred to
as a heartbeat. There's a standby node waiting to take over
should it stop receiving this signal.
I think only knowing that it has failed is not enough. Because
the process is landing, and other CPU should know where the
process is left. With that heatbeat signal, only option is that
all sensor information must be sent both CPUs continuously and
sensor values should be enough about what next step to be
taken. Then I think it can continue the process flawlessly.
I don't have experience with, or much knowledge of, these kinds
of systems; I'm merely aware of the concepts. The process of one
system taking over when another system fails is called failover
[1]. Depending on the requirements, the system could be designed
so the standby node continues from the last successful state of
the failed node [2].
To quote the page on Wikipedia [2], "Most importantly, the
application must store as much of its state on non-volatile
shared storage as possible. Equally important is the ability to
restart on another node at the last state before failure using
the saved state from the shared storage."
I would consider it likely that both systems run in conjunction,
but the primary system is in control and the backup system merely
"observes", ready to take over in an instant as soon as it no
longer detects a heartbeat.
[1] https://en.wikipedia.org/wiki/Failover
[2]
https://en.wikipedia.org/wiki/High-availability_cluster#Application_design_requirements
