Is it safe for read-only indices to have zero replicas?

[walid]

We have date-partitioned indices (by month), previous months indices are only used for searching, but still hold important data.

• Would it be safe to remove the replicas for those old indices in terms of data integrity?
• Would there be any search performance penalty by doing so?

[Christian_Dahlqvist]

If you run without replicas for any index loss of a node will result in the shards held solely by that node to be lost. You may have to restore from backups. If nodes on the other hand are temporarily unavailable, e.g. due to long GC, you may get partial or severely delayed responses. Any search will also hit a larger portion of shards on each node if you have no replicas, which can impact search performance.