Minimum number of replica shard ? (do we need 2 replica for recovery?)

[dan_kim]

Hello !

our company is trying to building a new Elasticsearch for service and We are studying ES
for a proper service .

And I have a question about replica shard.
Should we set our replica shard more than 2?

Here is reason for asking this question

1. if there is too much shard, indexing and updating performance go bad.
2. many case in distributed system, making 2 replica (slave) is recommended for recovery.

here is our dillema, I think if we want to get safety, we should set our replica as 2, but if we do so, our indexing rate would go bad ( our index is almost 2000 )

But im not sure that we should set our replica as 2 (maybe 1 replica casue no problem )

[warkolm (Mark Walkom)]

1. Have you tested that?
2. There's no such thing as a slave in Elasticsearch

@dan_kim

here is our dillema, I think if we want to get safety, we should set our replica as 2, but if we do so, our indexing rate would go bad ( our index is almost 2000 )

What are you protecting from?
How do you know your indexing rate would go bad?

[Christian_Dahlqvist]

@dan_kim

if there is too much shard, indexing and updating performance go bad.

The more replicas you have the more work is required in the cluster for indexing and updates, so increasing the number of replicas indeed tend to reduce indexing/update throughput.

@dan_kim

many case in distributed system, making 2 replica (slave) is recommended for recovery.

This depends on the size of your cluster, how many availability zones you have and what level of resiliency you require. If you only need to be able to handle one node going down having a primary ad a single replica is often sufficient as Elasticsearch will reallocate a replica to another node if one is lost.