# How to rebalance primary shards on elastic cluster **URL:** \ **Category:** Elasticsearch\ **Created:** [April 9, 2019, 3:28pm UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060 "2019-04-09T15:28:16Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![oraclept](https://avatars.discourse-cdn.com/v4/letter/o/ce73a5/32.png) [@oraclept](https://discuss.elastic.co/u/oraclept)\ **Post date:** [April 9, 2019, 3:28pm UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060/1 "2019-04-09T15:28:16Z")
Hi - How can I rebalance primary shards on elasticsearch cluster? Below are primary shards allocation on 5 node elastic cluster. _We are seeing huge spikes on node4_ ``` 6 node1 38 node2 116 node3 162 node4 35 node5 ``` Replica shards ``` 169 node1 136 node2 59 node3 12 node4 140 node5 ``` When we look at all shards allocation on cluster nodes it looks balanced. ``` 175 node1 174 node2 175 node3 174 node4 175 node5 ``` I modified below it did not do anything. ``` cluster.routing.rebalance.enable to primaries cluster.routing.allocation.allow_rebalance to always. ``` I want to balance shards so that all nodes gets equal number of primary shards if possible on index level. We have around 60 indicies and each index having varying shard numbers. ---
**Author:** ![andrese](https://avatars.discourse-cdn.com/v4/letter/a/8dc957/32.png) [@andrese](https://discuss.elastic.co/u/andrese)\ **Post date:** [April 16, 2019, 10:20am UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060/2 "2019-04-16T10:20:23Z")
++ Expanding this question, with our use case, where we justify why we also consider that primary sahrd rebalancing is needed. In other threads this has been answered as not being necessary saying that primary balancing wont matter that much as primary and replica shards should be carrying equivalents loads. But building over @oraclept question, we have a use case where this is not true, and primaries are doing a much harder work, so if primaries arenot balanced some nodes can be completely overloaded while others are idle. Consider for instance a heavy index scenario, where indexing involves upserting (and sadly upserting cant be avoided in our case). As part of indexing, primary shards have to query the data and do the upsert logic, and later propagate the changes to the replicas. Replicas dont need to execute the upserting logic, resulting in that primaries work is much heavier than replicas, so that the workload on the cluster is as unbalanced as the primary shards are. So, we also wonder ifs it possible to have primaries balanced on a cluster, automatically or executing some kind of procedure...? ---
**Author:** ![DavidTurner](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/davidturner/32/22453_2.png) [@DavidTurner](https://discuss.elastic.co/u/DavidTurner)\ **Post date:** [April 16, 2019, 11:46am UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060/3 "2019-04-16T11:46:05Z")
This question does come up occasionally, and you're right that in most cases it's not necessary. However, it is also true that an update-heavy workload will consume more resources on the primary than on a replica (and conversely this is the only kind of workload that would cause such an imbalance). There is no simple way to do primary balancing. The allocator doesn't really distinguish between primaries and replicas, and there is no API to promote a specific replica to a primary. You can try using the [cluster reroute API](https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster-reroute.html) to move a primary somewhere else, or to cancel the allocation of a primary (which will then promote a replica) but it doesn't cover all the cases. I can't see an issue in Github specifically requesting this feature. Could you open one? There is no simple way to achieve this with today's allocator, but it'd be good to start to quantify the support for this feature there. ---
**Author:** ![andrese](https://avatars.discourse-cdn.com/v4/letter/a/8dc957/32.png) [@andrese](https://discuss.elastic.co/u/andrese)\ **Post date:** [April 16, 2019, 1:43pm UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060/4 "2019-04-16T13:43:35Z")
Thanks for the quick response, Sure, I will open a issue in Github, as a new feature request and will also try to push it through our supported subscriptions. The cluster reroute API workaround is exactly what we are using in some clusters (moving replicas and a primaries in the same reroute transaction, to a node with less primaries). But we have found 2 main drawbacks that makes this solution insufficient: 1. Its a resource heavy operation, as shard data has to be moved arround. 2. We need to have spare node that won't host any shard from the affected index (we cannot swap a replica and a primary between 2 nodes without using an intermediate node). ---
**Author:** ![andrese](https://avatars.discourse-cdn.com/v4/letter/a/8dc957/32.png) [@andrese](https://discuss.elastic.co/u/andrese)\ **Post date:** [April 25, 2019, 5:13pm UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060/5 "2019-04-25T17:13:53Z")
Opened [issue #41543](https://github.com/elastic/elasticsearch/issues/41543). ---
**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\ **Post date:** [May 23, 2019, 5:19pm UTC](https://discuss.elastic.co/t/how-to-rebalance-primary-shards-on-elastic-cluster/176060/6 "2019-05-23T17:19:45Z")
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