105 lines
4.2 KiB
Plaintext
105 lines
4.2 KiB
Plaintext
[[delayed-allocation]]
|
|
=== Delaying allocation when a node leaves
|
|
|
|
When a node leaves the cluster for whatever reason, intentional or otherwise,
|
|
the master reacts by:
|
|
|
|
* Promoting a replica shard to primary to replace any primaries that were on the node.
|
|
* Allocating replica shards to replace the missing replicas (assuming there are enough nodes).
|
|
* Rebalancing shards evenly across the remaining nodes.
|
|
|
|
These actions are intended to protect the cluster against data loss by
|
|
ensuring that every shard is fully replicated as soon as possible.
|
|
|
|
Even though we throttle concurrent recoveries both at the
|
|
<<recovery,node level>> and at the <<shards-allocation,cluster level>>, this
|
|
``shard-shuffle'' can still put a lot of extra load on the cluster which
|
|
may not be necessary if the missing node is likely to return soon. Imagine
|
|
this scenario:
|
|
|
|
* Node 5 loses network connectivity.
|
|
* The master promotes a replica shard to primary for each primary that was on Node 5.
|
|
* The master allocates new replicas to other nodes in the cluster.
|
|
* Each new replica makes an entire copy of the primary shard across the network.
|
|
* More shards are moved to different nodes to rebalance the cluster.
|
|
* Node 5 returns after a few minutes.
|
|
* The master rebalances the cluster by allocating shards to Node 5.
|
|
|
|
If the master had just waited for a few minutes, then the missing shards could
|
|
have been re-allocated to Node 5 with the minimum of network traffic. This
|
|
process would be even quicker for idle shards (shards not receiving indexing
|
|
requests) which have been automatically <<indices-synced-flush,sync-flushed>>.
|
|
|
|
The allocation of replica shards which become unassigned because a node has
|
|
left can be delayed with the `index.unassigned.node_left.delayed_timeout`
|
|
dynamic setting, which defaults to `1m`.
|
|
|
|
This setting can be updated on a live index (or on all indices):
|
|
|
|
[source,js]
|
|
------------------------------
|
|
PUT /_all/_settings
|
|
{
|
|
"settings": {
|
|
"index.unassigned.node_left.delayed_timeout": "5m"
|
|
}
|
|
}
|
|
------------------------------
|
|
// AUTOSENSE
|
|
|
|
With delayed allocation enabled, the above scenario changes to look like this:
|
|
|
|
* Node 5 loses network connectivity.
|
|
* The master promotes a replica shard to primary for each primary that was on Node 5.
|
|
* The master logs a message that allocation of unassigned shards has been delayed, and for how long.
|
|
* The cluster remains yellow because there are unassigned replica shards.
|
|
* Node 5 returns after a few minutes, before the `timeout` expires.
|
|
* The missing replicas are re-allocated to Node 5 (and sync-flushed shards recover almost immediately).
|
|
|
|
NOTE: This setting will not affect the promotion of replicas to primaries, nor
|
|
will it affect the assignment of replicas that have not been assigned
|
|
previously. In particular, delayed allocation does not come into effect after a full cluster restart.
|
|
Also, in case of a master failover situation, elapsed delay time is forgotten
|
|
(i.e. reset to the full initial delay).
|
|
|
|
==== Cancellation of shard relocation
|
|
|
|
If delayed allocation times out, the master assigns the missing shards to
|
|
another node which will start recovery. If the missing node rejoins the
|
|
cluster, and its shards still have the same sync-id as the primary, shard
|
|
relocation will be cancelled and the synced shard will be used for recovery
|
|
instead.
|
|
|
|
For this reason, the default `timeout` is set to just one minute: even if shard
|
|
relocation begins, cancelling recovery in favour of the synced shard is cheap.
|
|
|
|
==== Monitoring delayed unassigned shards
|
|
|
|
The number of shards whose allocation has been delayed by this timeout setting
|
|
can be viewed with the <<cluster-health,cluster health API>>:
|
|
|
|
[source,js]
|
|
------------------------------
|
|
GET _cluster/health <1>
|
|
------------------------------
|
|
<1> This request will return a `delayed_unassigned_shards` value.
|
|
|
|
==== Removing a node permanently
|
|
|
|
If a node is not going to return and you would like Elasticsearch to allocate
|
|
the missing shards immediately, just update the timeout to zero:
|
|
|
|
|
|
[source,js]
|
|
------------------------------
|
|
PUT /_all/_settings
|
|
{
|
|
"settings": {
|
|
"index.unassigned.node_left.delayed_timeout": "0"
|
|
}
|
|
}
|
|
------------------------------
|
|
// AUTOSENSE
|
|
|
|
You can reset the timeout as soon as the missing shards have started to recover.
|