The track_io_timing command can give you clues about the database that is causing most of the load. However, there is more; pg_stat_bgwriter contains a lot of information about the system's I/O behavior:
test=# \d pg_stat_bgwriter
View "pg_catalog.pg_stat_bgwriter"
Column | Type |
-----------------------+--------------------------+
checkpoints_timed | bigint |
checkpoints_req | bigint |
checkpoint_write_time | double precision |
checkpoint_sync_time | double precision |
buffers_checkpoint | bigint |
buffers_clean | bigint |
maxwritten_clean | bigint |
buffers_backend | bigint |
buffers_backend_fsync | bigint |
buffers_alloc | bigint |
stats_reset | timestamp with time zone | Understanding...