I'm playing around a little bit with ES and now have an index with 35 Shards
and 1 Replica that I wanted to scale up to 20 replicas (on 21 nodes). While
performing the replication increase I have observed that it is overall very
slow. The servers are interconnected with bonded Gigabit network cables
within the same switch (giving 2Gbit/server). So I would assume that a
throughput of 100MByte/s between two nodes (!) would be achievable at least.
However, checking IO performance with iotop, the average "replication rate"
for the total cluster (!) is at approximately 20 MByte /sec which is awfully
I'm interested now in how elasticsearch does replicate from one server to
another. Is there a specific algorithm behind it and how does it work? Why
is the replication rate so slow? Are there any possibilities to increase the
Thanks already in advance a lot for your help!