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

[Christine Caulfield <ccaulfie@redhat.com>]

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