[Pacemaker] is ccs as racy as it feels?

Andrew Beekhof andrew at beekhof.net
Tue Dec 10 19:48:07 EST 2013

On 10 Dec 2013, at 11:31 pm, Brian J. Murrell <brian at interlinx.bc.ca> wrote:

> On Tue, 2013-12-10 at 10:27 +0000, Christine Caulfield wrote: 
>> Sadly you're not wrong.
> That's what I was afraid of.
>> But it's actually no worse than updating 
>> corosync.conf manually,
> I think it is...
>> in fact it's pretty much the same thing,
> Not really.  Updating corosync.conf on any given node means only having
> to write that file on that node.  There is no cluster-wide
> synchronization needed

Approximately speaking, cman takes cluster.conf and generates an in-memory corosync.conf equivalent to be passed to corosync.
So anything that could be done by editing corosync.conf should be possible with 'ccs -f ...', neither command results in any synchronisation or automatic update into the running process.

> and therefore no last-write-wins race so all
> nodes can do that in parallel.  Plus adding a new node means only having
> to update the corosync.conf on that new node (and starting up corosync
> of course) and corosync then does the job of telling it's peers about
> the new node rather than having to have the administrator go out and
> touch every node to inform them of the new member.

It sounds like this thread is less about cluster.conf vs. corosync.conf and more about autodiscovery vs. fixed node lists.

Chrissie: is there no way to use cman in autodiscovery mode (ie. with multicast/broadcast and learning about peers as they appear)?

> It's this removal of node auto-discovery and changing it to an operator
> task that is really complicating the workflow.  Granted, it's not so
> much complicating it for a human operator who is naturally only
> single-threaded and mostly incapable of inducing the last-write-wins
> races.
> But when you are writing tools that now have to take what used to be a
> very capable multithreaded task, free of races and shove it down a
> single-threaded pipe/queue just to eliminate races, this is a huge step
> backwards in evolution.
>> so 
>> nothing is actually getting worse.
> It is though.  See above.
>> All the CIB information is still 
>> properly replicated.
> Yeah.  I understand/understood that.  Pacemaker's actual operations go
> mostly unchanged.  It's the cluster membership process that's gotten
> needlessly complicated and regressed in functionality.
>> The main difficulty is in safely replicating information that's needed 
>> to boot the system.
> Do you literally mean staring the system up?  I guess the use-case you
> are describing here is booting nodes from a clustered filesystem?  But
> what if you don't need that complication?  This process is being made
> more complicated to satisfy only a subset of the use-cases.
>> In general use we've not found it to be a huge problem (though, I'm 
>> still not keen on it either TBH) because most management is done by one 
>> person from one node.
> Indeed.  As I said above, WRT to single-threaded operators.  But when
> you are writing a management system on top of all of this, which
> naturally wants to be multi-threaded (because scalable systems avoid
> bottlenecking through single choke points) and was able to be
> multithreaded when it was just corosync.conf, having to choke everything
> back down into a single thread just sucks.
>> There is not really any concept of nodes trying to 
>> "add themselves" to a cluster, it needs to be done by a person - which 
>> maybe what you're unhappy with.
> Yes, not so much "add themselves" but allowed to be added, in parallel
> without fear of racing.
> This ccs tool wouldn't be so bad if it operated more like the CIB where
> modifications were replicated automatically and properly locked so that
> modifications could be made anywhere on the cluster and all members got
> those modifications automatically rather than pushing off the work of
> locking, replication and serialization off onto the caller.
> b.
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