Losing a write is very different from having a fragile cluster. A fragile cluster implies that whole thing will fall apart, that it breaks easily. Writing at CL=ONE gives you a pretty damn stable cluster at the potential risk of losing a write that hasn't replicated (but has been ack'ed) which for a lot of people is preferable to downtime. CL=ONE gives you the *most stable* cluster you can have.
On Tue, Mar 29, 2016 at 12:57 AM Jacques-Henri Berthemet < jacques-henri.berthe...@genesys.com> wrote: > Because if you lose a node you have chances to lose some data forever if > it was not yet replicated. > > > > *--* > > *Jacques-Henri Berthemet* > > > > *From:* Jonathan Haddad [mailto:j...@jonhaddad.com] > *Sent:* vendredi 25 mars 2016 19:37 > > > *To:* user@cassandra.apache.org > *Subject:* Re: How many nodes do we require > > > > Why would using CL-ONE make your cluster fragile? This isn't obvious to > me. It's the most practical setting for high availability, which very much > says "not fragile". > > On Fri, Mar 25, 2016 at 10:44 AM Jacques-Henri Berthemet < > jacques-henri.berthe...@genesys.com> wrote: > > I found this calculator very convenient: > http://www.ecyrd.com/cassandracalculator/ > > Regardless of your other DCs you need RF=3 if you write at LOCAL_QUORUM, > RF=2 if you write/read at ONE. > > Obviously using ONE as CL makes your cluster very fragile. > -- > Jacques-Henri Berthemet > > > -----Original Message----- > From: Rakesh Kumar [mailto:rakeshkumar46...@gmail.com] > Sent: vendredi 25 mars 2016 18:14 > To: user@cassandra.apache.org > Subject: Re: How many nodes do we require > > On Fri, Mar 25, 2016 at 11:45 AM, Jack Krupansky > <jack.krupan...@gmail.com> wrote: > > It depends on how much data you have. A single node can store a lot of > data, > > but the more data you have the longer a repair or node replacement will > > take. How long can you tolerate for a full repair or node replacement? > > At this time, for a foreseeable future, size of data will not be > significant. So we can safely disregard the above as a decision > factor. > > > > > Generally, RF=3 is both sufficient and recommended. > > Are you telling a SimpleReplication topology with RF=3 > or NetworkTopology with RF=3. > > > taken from: > > > https://docs.datastax.com/en/cassandra/2.0/cassandra/architecture/architectureDataDistributeReplication_c.html > > " > Three replicas in each data center: This configuration tolerates > either the failure of a one node per replication group at a strong > consistency level of LOCAL_QUORUM or multiple node failures per data > center using consistency level ONE." > > In our case, with only 3 nodes in each DC, wouldn't a RF=3 effectively > mean ALL. > > I will state our requirement clearly: > > If we are going with six nodes (3 in each DC), we should be able to > write even with a loss of one DC and loss of one node of the surviving > DC. I am open to hearing what compromise we have to do with the reads > during the time a DC is down. For us write is critical, more than > reads. > > May be this is not possible with 6 nodes, and requires more. Pls advise. > >
