r/m populate_io_cache_on_flush from yaml (per-table since #4694)

conf/cassandra.yaml

@@ -57,11 +57,6 @@ max_hints_delivery_threads: 2
 # reduced proportionally to the number of nodes in the cluster.
 batchlog_replay_throttle_in_kb: 1024
 
-# The following setting populates the page cache on memtable flush and compaction
-# WARNING: Enable this setting only when the whole node's data fits in memory.
-# Defaults to: false
-# populate_io_cache_on_flush: false
-
 # Authentication backend, implementing IAuthenticator; used to identify users
 # Out of the box, Cassandra provides org.apache.cassandra.auth.{AllowAllAuthenticator,
 # PasswordAuthenticator}.

src/java/org/apache/cassandra/config/Config.java

@@ -168,7 +168,7 @@ public class Config
     public volatile int row_cache_save_period = 0;
     public int row_cache_keys_to_save = Integer.MAX_VALUE;
     public String row_cache_provider = SerializingCacheProvider.class.getSimpleName();
-    public boolean populate_io_cache_on_flush = false;
+    public boolean populate_io_cache_on_flush = false; // ignored! see CASSANDRA-4694
 
     public boolean inter_dc_tcp_nodelay = true;
 
@@ -204,7 +204,6 @@ public class Config
         periodic,
         batch
     }
-
     public static enum InternodeCompression
     {
         all, none, dc