Change concurrent_compactors defaults

patch by jbellis; reviewed by bes for CASSANDRA-7139

CHANGES.txt

@@ -1,4 +1,5 @@
 2.1.0-rc1
+ * Change concurrent_compactors defaults (CASSANDRA-7139)
  * Add PowerShell Windows launch scripts (CASSANDRA-7001)
  * Make commitlog archive+restore more robust (CASSANDRA-6974)
  * Fix marking commitlogsegments clean (CASSANDRA-6959)

conf/cassandra.yaml

@@ -500,8 +500,8 @@ in_memory_compaction_limit_in_mb: 64
 # slowly or too fast, you should look at
 # compaction_throughput_mb_per_sec first.
 #
-# concurrent_compactors defaults to the number of cores.
-# Uncomment to make compaction mono-threaded, the pre-0.8 default.
+# concurrent_compactors defaults to the smaller of (number of disks,
+# number of cores), with a minimum of 2 and a maximum of 8.
 #concurrent_compactors: 1
 
 # Throttles compaction to the given total throughput across the entire

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

@@ -132,7 +132,7 @@ public class Config
     public Integer column_index_size_in_kb = 64;
     public Integer batch_size_warn_threshold_in_kb = 5;
     public Integer in_memory_compaction_limit_in_mb = 64;
-    public Integer concurrent_compactors = FBUtilities.getAvailableProcessors();
+    public Integer concurrent_compactors;
     public volatile Integer compaction_throughput_mb_per_sec = 16;
 
     public Integer max_streaming_retries = 3;

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

@@ -453,7 +453,7 @@ public class DatabaseDescriptor
         }
 
         if (conf.concurrent_compactors == null)
-            conf.concurrent_compactors = FBUtilities.getAvailableProcessors();
+            conf.concurrent_compactors = Math.min(8, Math.max(2, Math.min(FBUtilities.getAvailableProcessors(), conf.data_file_directories.length)));
 
         if (conf.concurrent_compactors <= 0)
             throw new ConfigurationException("concurrent_compactors should be strictly greater than 0");