Default commitlog_sync_batch_window_in_ms changed to 2ms

patch by jbellis; reveiwed by bes for CASSANDRA-9504

CHANGES.txt

@@ -1,4 +1,5 @@
 2.1.6
+ * Default commitlog_sync_batch_window_in_ms changed to 2ms (CASSANDRA-9504)
  * Fix empty partition assertion in unsorted sstable writing tools (CASSANDRA-9071)
  * Ensure truncate without snapshot cannot produce corrupt responses (CASSANDRA-9388) 
  * Consistent error message when a table mixes counter and non-counter

NEWS.txt

@@ -15,6 +15,7 @@ using the provided 'sstableupgrade' tool.
 
 2.1.5
 =====
+
 Upgrading
 ---------
     - The option to omit cold sstables with size tiered compaction has been
@@ -24,8 +25,13 @@ Upgrading
 
 2.1.4
 =====
+
+Upgrading
+---------
 The default JMX config now listens to localhost only. You must enable 
 the other JMX flags in cassandra-env.sh manually.
+
+
 2.1.3
 =====
 
@@ -37,6 +43,7 @@ Upgrading
     - Incremental replacement of compacted SSTables has been disabled for this
       release.
 
+
 2.1.2
 =====
 
@@ -45,6 +52,7 @@ Upgrading
     - Nothing specific to this release, but please see 2.1 if you are upgrading
       from a previous version.
 
+
 2.1.1
 =====
 
@@ -59,6 +67,7 @@ New features
      reason you want to disable it pass, the following system property
      -Dcassandra.native.epoll.enabled=false
 
+
 2.1
 ===
 
@@ -106,6 +115,11 @@ New features
 
 Upgrading
 ---------
+   - commitlog_sync_batch_window_in_ms behavior has changed from the
+     maximum time to wait between fsync to the minimum time.  We are 
+     working on making this more user-friendly (see CASSANDRA-9533) but in the
+     meantime, this means 2.1 needs a much smaller batch window to keep
+     writer threads from starving.  The suggested default is now 2ms.
    - Rolling upgrades from anything pre-2.0.7 is not supported. Furthermore
      pre-2.0 sstables are not supported. This means that before upgrading
      a node on 2.1, this node must be started on 2.0 and
@@ -126,6 +140,7 @@ Upgrading
      has been deprecated.
    - support for supercolumns has been removed from json2sstable
 
+
 2.0.11
 ======
 

conf/cassandra.yaml

@@ -226,13 +226,16 @@ counter_cache_save_period: 7200
 # saved_caches_directory: /var/lib/cassandra/saved_caches
 
 # commitlog_sync may be either "periodic" or "batch." 
+# 
 # When in batch mode, Cassandra won't ack writes until the commit log
-# has been fsynced to disk.  It will wait up to
+# has been fsynced to disk.  It will wait
-# commitlog_sync_batch_window_in_ms milliseconds for other writes, before
+# commitlog_sync_batch_window_in_ms milliseconds between fsyncs.
-# performing the sync.
+# This window should be kept short because the writer threads will
+# be unable to do extra work while waiting.  (You may need to increase
+# concurrent_writes for the same reason.)
 #
 # commitlog_sync: batch
-# commitlog_sync_batch_window_in_ms: 50
+# commitlog_sync_batch_window_in_ms: 2
 #
 # the other option is "periodic" where writes may be acked immediately
 # and the CommitLog is simply synced every commitlog_sync_period_in_ms