Thanks, Rahul.
But in your example, at the same time loss of Node3 and Node6 leads to loss of
ranges N, C, J at consistency level QUORUM.
As far as I understand in case vnodes > N_nodes_in_cluster and
endpoint_snitch=SimpleSnitch, since:
1) "secondary" replicas are placed on two nodes 'next' to the node responsible
for a range (in case of RF=3)
2) there are a lot of vnodes on each node
3) ranges are evenly distribusted between vnodes in case of SimpleSnitch,
we get all physical nodes (servers) having mutually adjacent token rages.
Is it correct?
At least in case of my real-world ~50-nodes cluster with nvodes=256, RF=3 for
this command:
nodetool ring | grep '^<ip-prefix>' | awk '{print $1}' | uniq | grep -B2 -A2
'<ip_of_a_node>' | grep -v '<ip_of_a_node>' | grep -v '^--' | sort | uniq | wc
-l
returned number which equals to Nnodes -1, what means that I can't switch off 2
nodes at the same time w/o losing of some keyrange for CL=QUORUM.
Thanks,
Kyrill
________________________________
From: Rahul Neelakantan <ra...@rahul.be>
Sent: Monday, January 15, 2018 5:20:20 PM
To: user@cassandra.apache.org
Subject: Re: vnodes: high availability
Not necessarily. It depends on how the token ranges for the vNodes are assigned
to them. For example take a look at this diagram
http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/architecture/architectureDataDistributeDistribute_c.html
In the vNode part of the diagram, you will see that Loss of Node 3 and Node 6,
will still not have any effect on Token Range A. But yes if you lose two nodes
that both have Token Range A assigned to them (Say Node 1 and Node 2), you will
have unavailability with your specified configuration.
You can sort of circumvent this by using the DataStax Java Driver and having
the client recognize a degraded cluster and operate temporarily in downgraded
consistency mode
http://docs.datastax.com/en/latest-java-driver-api/com/datastax/driver/core/policies/DowngradingConsistencyRetryPolicy.html
- Rahul
On Mon, Jan 15, 2018 at 10:04 AM, Kyrylo Lebediev
<kyrylo_lebed...@epam.com<mailto:kyrylo_lebed...@epam.com>> wrote:
Hi,
Let's say we have a C* cluster with following parameters:
- 50 nodes in the cluster
- RF=3
- vnodes=256 per node
- CL for some queries = QUORUM
- endpoint_snitch = SimpleSnitch
Is it correct that 2 any nodes down will cause unavailability of a keyrange at
CL=QUORUM?
Regards,
Kyrill
