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https://issues.apache.org/jira/browse/CASSANDRA-14265?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16382011#comment-16382011
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Kenneth Brotman commented on CASSANDRA-14265:
---------------------------------------------
>From DataStax at
>[https://docs.datastax.com/en/dse/5.1/dse-admin/datastax_enterprise/config/configVnodes.html]
h1. Virtual node (vnode) configuration
A description of virtual nodes (vnodes) and how to use them in different types
of datacenters. Also steps for disabling vnodes.
Virtual nodes simplify many tasks in DataStax Enterprise, such as eliminating
the need to determine the partition range (calculate and assign tokens),
rebalancing the cluster when adding or removing nodes, and replacing dead
nodes. For a complete description of virtual nodes and how they work, see
[Virtual
nodes|https://issues.apache.org/en/dse/5.1/dse-arch/datastax_enterprise/dbArch/archDataDistributeVnodesUsing.html].
DataStax Enterprise requires the same token architecture on all nodes in a
datacenter. The nodes must all be vnode-enabled or single-token architecture.
Across the entire cluster, datacenter architecture can vary. For example, a
single cluster with: * A transaction-only datacenter running OLTP.
* A single-token architecture analytics datacenter (no vnodes).
* A search datacenter with vnodes.
h2. Guidelines for using virtual nodes
Whether virtual nodes (vnodes) are enabled or disabled depends on the initial
[cassandra.yaml|#configVnodes__cassandrayaml] settings. There are two methods
of distributing token ranges. DataStax recommends using the allocation
algorithm. Use the same method on all systems in the datacenter.Allocation
algorithmOptimizes token range distribution between nodes and racks in the
datacenter based on the keyspace replication factor
(allocate_tokens_for_local_replication_factor) of the datacenter. Distributes
the token ranges proportionately using the num_tokens settings. All systems in
the datacenter should have the same {{num_token}} settings unless the systems
performance varies between systems. To distribute more of the workload to the
higher performance hardware, increase the number of tokens for those systems.
The allocation algorithm efficiently balances the workload using fewer tokens;
when systems are added to a datacenter, the algorithm maintains the balance.
Using a higher number of tokens more evenly distributes the workload, but also
significantly increases token management overhead.
CAUTION:
When adding multiple nodes to the cluster using the allocation algorithm,
ensure that nodes are added one at a time. If nodes are added concurrently, the
algorithm assigns the same tokens to different nodes.
DataStax recommends using *8* vnodes (tokens). This distributes the workload
between systems with a ~10% variance and has minimal impact on performance. Set
the number of vnode tokens ({{num_tokens}}) based on the workload distribution
requirements of the datacenter:
|Allocation algorithm workload distribution variance|
||Replication factor||4 vnode (tokens)||8 vnode (tokens)||64 vnode
(tokens)||128 vnode (tokens)||
|2|~17.5%|~12.5%|~3%|~1%|
|3|~14%|~10%|~2%|~1%|
|5|~11%|~7%|~1%|~1%|
h2. Enabling vnodes
In the [cassandra.yaml|#configVnodes__cassandrayaml] file:
# Uncomment num_tokens and set the required number of tokens.
# (Recommended) To use the allocation algorithm uncomment
allocate_tokens_for_local_replication_factor and set it to the target
replication factor for the keyspaces in the datacenter. If the replication
varies, alternate between the replication factor (RF) settings.
# Comment out the initial_token or leave unset.
To upgrade existing clusters to vnodes, see Enabling virtual nodes on an
existing production cluster.
h2. Disabling vnodes
Important: If you do not use vnodes, you must make sure that each node is
responsible for roughly an equal amount of data. To ensure that each node is
responsible for an equal amount of data, assign each node an initial-token
value and calculate the tokens for each datacenter as described in [Generating
tokens|https://issues.apache.org/jira/production/calcTokens.html]. # In the
[cassandra.yaml|#configVnodes__cassandrayaml] file:
## Comment out the num_tokens and allocate_tokens_for_local_replication_factor.
## Uncomment the initial_token and set it to 1 or to the value of a [generated
token|https://issues.apache.org/jira/production/calcTokens.html] for a
multi-node cluster.
cassandra.yaml
The location of the cassandra.yaml file depends on the type of installation:
|Package installations
Installer-Services installations|/etc/dse/cassandra/cassandra.yaml|
|Tarball installations
Installer-No Services
installations|installation_location/resources/cassandra/conf/cassandra.yaml|
*
**
** [Configuring Virtual
Nodes|https://issues.apache.org/datastax_enterprise/config/configVnodeTOC.html]
Topics about setting up and enabling virtual nodes (vnodes).
*** [Virtual node (vnode)
configuration|https://issues.apache.org/datastax_enterprise/config/configVnodes.html]
A description of virtual nodes (vnodes) and how to use them in different types
of datacenters. Also steps for disabling vnodes.
*** [Enabling virtual nodes on an existing production
cluster|https://issues.apache.org/datastax_enterprise/config/configVnodesProduction.html]
Steps and recommendations for enabling virtual nodes (vnodes) on an existing
production cluster.
** [Logging
configuration|https://issues.apache.org/datastax_enterprise/config/configLogginTOC.html]
Topics about changing logging locations, setting logging levels, archiving
logs, and CDC logging.
[Configuring Virtual
Nodes|https://issues.apache.org/datastax_enterprise/config/configVnodeTOC.html]
[Enabling virtual nodes on an existing production
cluster|https://issues.apache.org/datastax_enterprise/config/configVnodesProduction.html]
> Add explanation of vNodes to online documentation
> -------------------------------------------------
>
> Key: CASSANDRA-14265
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14265
> Project: Cassandra
> Issue Type: Improvement
> Components: Documentation and Website
> Reporter: Kenneth Brotman
> Priority: Major
>
> A lot of inquiries on the mailing list about how vNodes work and how to set
> configuration properly. We should add an explanation to the documentation.
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