[Linux-HA] uname -n configuration requirement

[Lars Marowsky-Bree]

On 2005-02-28T11:53:13, Guochun Shi <gshi at ncsa.uiuc.edu> wrote:

> >Well, we can of course always just initialize is to 32, 64 or
> >whatever, or does it cost more than just a couple of bytes? And it
> >might also be possible to increase it at run time.
> Yes, I believe we can make it work easily for this part. More
> challenge is how to compute quorum.

CCM needs to exchange information about nodes which have been added or
removed as part of the concensus computation, too. Only if all nodes
agree on the list of candidates they can proceed.

So, they need to exchange a checksum of said list first (because in 99%
of all cases, the lists will have been exchanged already and be synced).

If this checksum does NOT match, each node must enumerate the nodes it
knows about to the transition leader. Alas, all of them, including the
removed ones. The transition leader than needs to merge them all, and
rebroadcasts to all cluster members.

(This can probably be optimized by the transition leader, or whatever
the CCM calls it again, only requesting each list associated with each
checksum once.)

Of course, this algorithm is not perfect; if the admin removes / adds
nodes a cluster split, the quorum computation can be wrong, because the
other partitions can't know. This however is an unsolvable problem and
the admin ought to be whacked for it.

We _could_ probably add some safeguards against this in the management
tool, by suggesting the node list shouldn't be changed if not all nodes
are up and joined.


Sincerely,
    Lars Marowsky-Brée <lmb at suse.de>

-- 
High Availability & Clustering
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