Shard states

Before a shard can be used, it goes through an INITIALIZING state. shards can show you which ones.

% curl -XPUT 192.168.56.20:9200/_settings -d'{"number_of_replicas":1}'
{"acknowledged":true}
% curl 192.168.56.20:9200/_cat/shards
wiki1 0 p STARTED      3014 31.1mb 192.168.56.10 Stiletto
wiki1 0 r INITIALIZING    0 14.3mb 192.168.56.30 Frankie Raye
wiki1 1 p STARTED      3013 29.6mb 192.168.56.30 Frankie Raye
wiki1 1 r INITIALIZING    0 13.1mb 192.168.56.20 Commander Kraken
wiki1 2 r INITIALIZING    0   14mb 192.168.56.10 Stiletto
wiki1 2 p STARTED      3973 38.1mb 192.168.56.20 Commander Kraken

If a shard cannot be assigned, for example you’ve overallocated the number of replicas for the number of nodes in the cluster, they will remain UNASSIGNED.

% curl -XPUT 192.168.56.20:9200/_settings -d'{"number_of_replicas":3}'
% curl 192.168.56.20:9200/_cat/health
1384316325 20:18:45 foo yellow 3 3 9 3 0 0 3
% curl 192.168.56.20:9200/_cat/shards
wiki1 0 p STARTED    3014 31.1mb 192.168.56.10 Stiletto
wiki1 0 r STARTED    3014 31.1mb 192.168.56.30 Frankie Raye
wiki1 0 r STARTED    3014 31.1mb 192.168.56.20 Commander Kraken
wiki1 0 r UNASSIGNED
wiki1 1 r STARTED    3013 29.6mb 192.168.56.10 Stiletto
wiki1 1 p STARTED    3013 29.6mb 192.168.56.30 Frankie Raye
wiki1 1 r STARTED    3013 29.6mb 192.168.56.20 Commander Kraken
wiki1 1 r UNASSIGNED
wiki1 2 r STARTED    3973 38.1mb 192.168.56.10 Stiletto
wiki1 2 r STARTED    3973 38.1mb 192.168.56.30 Frankie Raye
wiki1 2 p STARTED    3973 38.1mb 192.168.56.20 Commander Kraken
wiki1 2 r UNASSIGNED