Hi,
Yes, you will have 4 copies of your data, your nodes will be mirrors of each
other in effect.
R and W only control one thing; the number of replies we wait for before
returning your response. All N requests are made, in parallel, no matter what
setting for R or W you use. You're not saving I/O by changing it, you are just
modifying your latency (lower values of R and W will lower request latency) and
consistency (higher values of R and W will improve consistency, though nothing
delivers strong consistency in CouchDB).
Your understanding is not quite right, and so there neither are the inferences
made from that base.
B.
--
Robert Samuel Newson
rnew...@apache.org
On Mon, 11 Mar 2019, at 15:25, Vladimir Ralev wrote:
> Ah thanks a lot for the reply.
>
> The idea for n = 4 is both fault tolerance and performance. Since I have
> very few writes, I expect replication IO and view indexing IO to be minimal
> and I have no issues with temporary inconsistencies and conflicts.
>
> My understanding is that since there are very few writes, the 4 nodes will
> behave almost like 4 independent single nodes and will be able to serve the
> read requests independently without having to proxy to cluster peers and
> thus avoiding a great deal of extra network and disk IO.
>
> R=3 to me means 3 times the IO and thus 3 machines will be busy for the
> same read request instead of serving other requests. Which I understand is
> 3 times less performance from the cluster as a whole.
>
> If my understanding is correct, I imagine this would be a common use-case
> for couch?
>
> On Mon, Mar 11, 2019 at 4:58 PM Robert Newson <rnew...@apache.org> wrote:
>
> > r and w are no longer configurable from the config file by design. The
> > default is n/2+1 (so 3 in your case) unless you specify r or w as request
> > parameters.
> >
> > setting n = 4 for a 4 node cluster is very unusual, do you really need 4
> > full copies of your data?
> >
> > couchdb will also automatically lower both r and w if nodes are offline.
> >
> > The default of n=3, r=w=2 is appropriate in almost all cases as the right
> > balance between data safety and availability. Nothing you've said so far
> > suggests it would be good to deviate from those settings.
> >
> > --
> > Robert Samuel Newson
> > rnew...@apache.org
> >
> > On Mon, 11 Mar 2019, at 14:52, Vladimir Ralev wrote:
> > > Hi all,
> > >
> > > I am looking into running a 4-node couchdb 2.3 with this config in
> > > default.ini and I made sure no other config file override them:
> > > [cluster]
> > > q = 8
> > > n = 4
> > > r = 1
> > > w = 1
> > >
> > > But when i create a test DB and check the settings I get:
> > > curl -s couch01:5984/mytest1234 |jq . .... .... "cluster": { "q": 8,
> > "n": 4,
> > > "w": 3, "r": 3 },
> > >
> > > r and w settings are not respected and seem stuck to be the defaults.
> > >
> > > When I kill 3 of the machine and test reads and writes, they still work
> > > fine so it doesn't seem like the r and w are actually 3 either. I checked
> > > if the debug logs printed out the r and w anywhere to confirm what is
> > being
> > > configured or executed but there is nothing.
> > >
> > > It is unclear if r and w are active in this version of couch. I can see
> > > the
> > > they have been partially removed from the documentation
> > > https://docs.couchdb.org/en/master/cluster/theory.html as opposed to
> > > couchdb 2.0.0 original doc
> > >
> > https://web.archive.org/web/20160109122310/https://docs.couchdb.org/en/stable/cluster/theory.html
> > >
> > > Additionally curl -s couch01:5984/mytest1234/doc?r=3
> > > still works even if 3 out of the 4 nodes are dead which is unexpected per
> > > the quorum documentation here
> > > https://docs.couchdb.org/en/master/cluster/sharding.html#quorum
> > >
> > > My specific concern with r and w is that if r is 3 this means 3 times
> > more
> > > network and disk IO since it will have to read 3 times from remote
> > > machines. My use case really doesn't need this and performance will
> > suffer.
> > > This is a little hard to test so I was hopinh someone can shed some light
> > > on the current situation with r and w in couch 2.3.
> > >
> > > Thanks
> > >
> >
>
