There are different caching inner modules associated with an index. They
include filter and others.
The filter cache is responsible for caching the results of filters (used
in the query). The default implementation of a filter cache (and the one
recommended to use in almost all cases) is the node filter cache type.
The node filter cache may be configured to use either a percentage of
the total memory allocated to the process or an specific amount of
memory. All shards present on a node share a single node cache (thats
why its called node). The cache implements an LRU eviction policy:
when a cache becomes full, the least recently used data is evicted to
make way for new data.
The setting that allows one to control the memory size for the filter
cache is indices.cache.filter.size, which defaults to 10%. Note,
this is not an index level setting but a node level setting (can be
configured in the node configuration).
indices.cache.filter.size can accept either a percentage value, like
30%, or an exact value, like 512mb.