Make PeriodicCommitLogService.blockWhenSyncLagsNanos configurable

patch by jasobrown; reviewed by Jordan West for CASSANDRA-14580

CHANGES.txt

@@ -1,4 +1,5 @@
 4.0
+ * Make PeriodicCommitLogService.blockWhenSyncLagsNanos configurable (CASSANDRA-14580)
  * Improve logging in MessageInHandler's constructor (CASSANDRA-14576)
  * Set broadcast address in internode messaging handshake (CASSANDRA-14579)
  * Wait for schema agreement prior to building MVs (CASSANDRA-14571)

conf/cassandra.yaml

@@ -389,6 +389,10 @@ counter_cache_save_period: 7200
 commitlog_sync: periodic
 commitlog_sync_period_in_ms: 10000
 
+# When in periodic commitlog mode, the number of milliseconds to block writes
+# while waiting for a slow disk flush to complete.
+# periodic_commitlog_sync_lag_block_in_ms: 
+
 # The size of the individual commitlog file segments.  A commitlog
 # segment may be archived, deleted, or recycled once all the data
 # in it (potentially from each columnfamily in the system) has been

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

@@ -204,6 +204,7 @@ public class Config
     public int commitlog_segment_size_in_mb = 32;
     public ParameterizedClass commitlog_compression;
     public int commitlog_max_compression_buffers_in_pool = 3;
+    public Integer periodic_commitlog_sync_lag_block_in_ms;
     public TransparentDataEncryptionOptions transparent_data_encryption_options = new TransparentDataEncryptionOptions();
 
     public Integer max_mutation_size_in_kb;

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

@@ -1866,6 +1866,14 @@ public class DatabaseDescriptor
         return conf.commitlog_sync_period_in_ms;
     }
 
+    public static long getPeriodicCommitLogSyncBlock()
+    {
+        Integer blockMillis = conf.periodic_commitlog_sync_lag_block_in_ms;
+        return blockMillis == null
+               ? (long)(getCommitLogSyncPeriod() * 1.5)
+               : blockMillis;
+    }
+
     public static void setCommitLogSyncPeriod(int periodMillis)
     {
         conf.commitlog_sync_period_in_ms = periodMillis;

src/java/org/apache/cassandra/db/commitlog/PeriodicCommitLogService.java

@@ -17,11 +17,13 @@
  */
 package org.apache.cassandra.db.commitlog;
 
+import java.util.concurrent.TimeUnit;
+
 import org.apache.cassandra.config.DatabaseDescriptor;
 
 class PeriodicCommitLogService extends AbstractCommitLogService
 {
-    private static final long blockWhenSyncLagsNanos = (long) (DatabaseDescriptor.getCommitLogSyncPeriod() * 1.5e6);
+    private static final long blockWhenSyncLagsNanos = TimeUnit.MILLISECONDS.toNanos(DatabaseDescriptor.getPeriodicCommitLogSyncBlock());
 
     public PeriodicCommitLogService(final CommitLog commitLog)
     {