Why doesn't my cluster performance scale when I double the number of machines?

Okay Alex, thank you!

This is what I have in my cluster of 10 machines at the moment:

ip               port    cpu%  mem%  iops  net  class        roles
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.32.74     4500    1     6     6     0    log          log
                  4501    0     4     -     0    stateless
                  4502    0     3     -     0    stateless
                  4503    5     3     -     2    stateless    cluster_controller
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.33.171    4500    1     5     9     0    storage      storage
                  4501    1     4     9     0    storage      storage
                  4502    0     3     -     0    stateless
                  4503    1     3     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.33.172    4500    1     6     8     0    storage      storage
                  4501    1     4     8     0    storage      storage
                  4502    1     3     -     0    stateless
                  4503    1     2     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.34.155    4500    1     6     9     0    storage      storage
                  4501    1     4     9     0    storage      storage
                  4502    0     3     -     0    stateless
                  4503    1     3     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.37.131    4500    1     5     8     0    storage      storage
                  4501    1     4     8     0    storage      storage
                  4502    1     4     -     0    stateless
                  4503    3     3     -     0    stateless    master
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.39.85     4500    1     4     7     0    log          log
                  4501    0     4     -     0    stateless
                  4502    1     2     -     0    stateless
                  4503    1     2     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.42.96     4500    1     5     8     0    storage      storage
                  4501    1     4     8     0    storage      storage
                  4502    1     2     -     0    proxy        proxy
                  4503    1     3     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.44.149    4500    1     4     8     0    storage      storage
                  4501    1     4     8     0    storage      storage
                  4502    1     2     -     0    proxy        proxy
                  4503    0     2     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.46.120    4500    1     5     8     0    storage      storage
                  4501    1     4     8     0    storage      storage
                  4502    0     3     -     0    stateless
                  4503    0     3     -     0    stateless
---------------  ------  ----  ----  ----  ---  -----------  --------------------
 172.31.47.4      4500    1     6     8     0    storage      storage
                  4501    1     4     8     0    storage      storage
                  4502    1     3     -     0    stateless
                  4503    1     4     -     0    stateless    resolver

In your last comment Alex, are you suggesting that I need to prepare my database by pre-creating some shards? I’m definitely not doing anything special…

Thanks