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Re: [Cluster-devel] SCTP versus OpenAIS/corosync time-outs



On 31/10/09 00:20, Lars Marowsky-Bree wrote:
Hi all, David,

I'm contemplating SCTP versus OpenAIS/corosync. Is dlm_controld(.pcmk)
pro-actively informed if a single ring/link goes down, as to trigger
faster SCTP recovery - or is it left for SCTP to time out on its own and
proceed?

Corosync tells no-one, apart from syslog, if a link goes down. I imagine its possible for the CFG subsystem to inform applications of link state changes but it doesn't currently do so.


If the latter - is there a way to auto-tune the SCTP time-outs to make
sure the DLM doesn't stall longer than that? I'm wondering whether
there's any chance for higher-level time-outs, ie a monitor operation on
a filesystem-using service.

I imagine it's possible to tell SCTP the cman values for timeouts. It doesn't happen at the moment but perhaps it should. There is a lot of score for more auto-configuration of things in clustering I think.


RFC 5061 seems to support dynamic reconfiguration in such a fashion. If
I'm reading http://tools.ietf.org/html/rfc4960#page-87 correctly, SCTP
multi-homing is "active/passive", so there's some latency on the
fail-over at least. If several links go down at once, SCTP might try
them in sequence and pick the one surviving link last, incurring a large
latency.

No concurrently active transmission ("rrp_mode active") - I wonder if it
is possible to put SCTP into such an mode, or, vice-versa, if this means
the DLM might be better off directly opening several TCP connections on
its own (and using them all at once, simply discarding duplicate
messages)?

If you want to add TCP multi-homing code to the DLM, feel free. But it'll be complicated and messy I promise. And it seems pointless to reimplement all the sort of failover code that's already in SCTP for free.

I'm not sure what kind of problems exist, if any, but this may be a
worth-while thing to consider or at least contemplate. I welcome
feedback ;-)

To be honest, RRP & DLM/SCTP is not well tested or used. There are probably lots of things that could be done to improve it. In particular the failover aspect of it (the most important part of course) has probably not been tried under any sort of serious load ... though i could be wrong.


Chrissie


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