With cold caches it can be difficult to put load on ramoth without overloading it. The standard solution to this is pg_prewarm, but that requires 9.4.

Instead, this can be done with shell script as the user postgres

find "$(psql -d postgres -AXtqwc 'SHOW data_directory;')" -type f \
  | grep "$(psql -d osm2pgsql -AXtqwc 'SELECT pg_relation_filepath(r.oid) || '\''.*'\'' FROM pg_class r WHERE r.relname='\''planet_osm_polygon_index'\')" \
  | xargs -i cat {} > /dev/null

Replace osm2pgsql and planet_osm_polygon_index with appropriate db and index names. To do multiple indexes, run the command multiple times.

The shell script turns into a command like

find /var/lib/postgresql/9.5/main -type f | grep "base/5917088/8222095.*" | xargs -i cat {} > /dev/null

which then reads the /var/lib/postgresql/9.5/main/base/5917088/8222095, /var/lib/postgresql/9.5/main/base/5917088/8222095.1, /var/lib/postgresql/9.5/main/base/5917088/8222095.2, etc files.

This assumes that cat f > /dev/null reads the file, so it doesn't work on Solaris which uses mmap for cat.

With 9.4 or later, this is all done with CREATE EXTENSION pg_prewarm; SELECT pg_prewarm('planet_osm_polygon_index', 'prefetch');'