Why not use NetworkTopology and specify each region as a ‘DC’ ?
Setup a snitch (propertyFile or Gossip, or even the EC2Region one) to list out
which nodes are in which DC.
Then when creating the Keyspace, specify NetworkTopology, with RF1 in each DC
/ Rack.
Ie.
CREATE KEYSPACE fred WITH replication = {'class': 'NetworkTopologyStrategy',
'DC2': '1', 'DC3': '1', 'DC1': '1'};
Regards
Mark Farnan
From: William Oberman [mailto:ober...@civicscience.com]
Sent: Tuesday, May 13, 2014 11:11 PM
To: user@cassandra.apache.org
Subject: NTS, vnodes and 0% chance of data loss
I found this:
http://mail-archives.apache.org/mod_mbox/cassandra-user/201404.mbox/%3ccaeduwd1erq-1m-kfj6ubzsbeser8dwh+g-kgdpstnbgqsqc...@mail.gmail.com%3E
I read the three referenced cases. In addition, case 4123 references:
http://www.mail-archive.com/dev@cassandra.apache.org/msg03844.html
And even though I *think* I understand all of the issues now, I still want to
double check...
Assumptions:
-A cluster using NTS with options [DC:3]
-Physical layout = In DC, 3 nodes/rack for a total of 9 nodes
No vnodes: I could do token selection using ideas from case 3810 such that each
rack has one replica. At this point, my "0% chance of data loss" scenarios are:
1.) Failure of two nodes at random
2.) Failure of 2 racks (6 nodes!)
Vnodes: my "0% chance of data loss" scenarios are:
1.) Failure of two nodes at random
Which means a rack failure (3 nodes) has a non-zero chance of data failure
(right?).
To get specific, I'm in AWS, so racks ~= "availability zones". In the years
I've been in AWS, I've seen several occasions of "single zone downtimes", and
one time of "single zone catastrophic loss". E.g. for AWS I feel like you
*have* to plan for a single zone failure, and in terms of "safety first" you
*should* plan for two zone failures.
To mitigate this data loss risk seems rough for vnodes, again if I'm
understanding everything correctly:
-To ensure 0% data loss for one zone => I need RF=4
-To ensure 0% data loss for two zones => I need RF=7
I'd really like to use vnodes, but RF=7 is crazy.
To reiterate what I think is the core idea of this message:
1.) for vnodes 0% data loss => RF=(# of allowed failures at once)+1
2.) racks don't change the above equation at all
will
