src/java/org/apache/cassandra/db/commitlog/AbstractCommitLogSegmentManager.java - cassandra - Git at Google

 /*
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information
  * regarding copyright ownership.  The ASF licenses this file
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 package org.apache.cassandra.db.commitlog;

 import java.io.File;
 import java.io.IOException;
 import java.util.*;
 import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.BooleanSupplier;

 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.util.concurrent.*;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

 import net.nicoulaj.compilecommand.annotations.DontInline;
 import org.apache.cassandra.concurrent.NamedThreadFactory;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.config.Schema;
 import org.apache.cassandra.db.*;
 import org.apache.cassandra.utils.*;
 import org.apache.cassandra.utils.concurrent.WaitQueue;

 import static org.apache.cassandra.db.commitlog.CommitLogSegment.Allocation;

 /**
  * Performs eager-creation of commit log segments in a background thread. All the
  * public methods are thread safe.
  */
 public abstract class AbstractCommitLogSegmentManager
 {
     static final Logger logger = LoggerFactory.getLogger(AbstractCommitLogSegmentManager.class);

     /**
      * Segment that is ready to be used. The management thread fills this and blocks until consumed.
      *
      * A single management thread produces this, and consumers are already synchronizing to make sure other work is
      * performed atomically with consuming this. Volatile to make sure writes by the management thread become
      * visible (ordered/lazySet would suffice). Consumers (advanceAllocatingFrom and discardAvailableSegment) must
      * synchronize on 'this'.
      */
     private volatile CommitLogSegment availableSegment = null;

     private final WaitQueue segmentPrepared = new WaitQueue();

     /** Active segments, containing unflushed data. The tail of this queue is the one we allocate writes to */
     private final ConcurrentLinkedQueue<CommitLogSegment> activeSegments = new ConcurrentLinkedQueue<>();

     /**
      * The segment we are currently allocating commit log records to.
      *
      * Written by advanceAllocatingFrom which synchronizes on 'this'. Volatile to ensure reads get current value.
      */
     private volatile CommitLogSegment allocatingFrom = null;

     final String storageDirectory;

     /**
      * Tracks commitlog size, in multiples of the segment size.  We need to do this so we can "promise" size
      * adjustments ahead of actually adding/freeing segments on disk, so that the "evict oldest segment" logic
      * can see the effect of recycling segments immediately (even though they're really happening asynchronously
      * on the manager thread, which will take a ms or two).
      */
     private final AtomicLong size = new AtomicLong();

     private Thread managerThread;
     protected final CommitLog commitLog;
     private volatile boolean shutdown;
     private final BooleanSupplier managerThreadWaitCondition = () -> (availableSegment == null && !atSegmentBufferLimit()) || shutdown;
     private final WaitQueue managerThreadWaitQueue = new WaitQueue();

     private static final SimpleCachedBufferPool bufferPool =
         new SimpleCachedBufferPool(DatabaseDescriptor.getCommitLogMaxCompressionBuffersInPool(), DatabaseDescriptor.getCommitLogSegmentSize());

     AbstractCommitLogSegmentManager(final CommitLog commitLog, String storageDirectory)
     {
         this.commitLog = commitLog;
         this.storageDirectory = storageDirectory;
     }

     void start()
     {
         // The run loop for the manager thread
         Runnable runnable = new WrappedRunnable()
         {
             public void runMayThrow() throws Exception
             {
                 while (!shutdown)
                 {
                     try
                     {
                         assert availableSegment == null;
                         logger.trace("No segments in reserve; creating a fresh one");
                         availableSegment = createSegment();
                         if (shutdown)
                         {
                             // If shutdown() started and finished during segment creation, we are now left with a
                             // segment that no one will consume. Discard it.
                             discardAvailableSegment();
                             return;
                         }

                         segmentPrepared.signalAll();
                         Thread.yield();

                         if (availableSegment == null && !atSegmentBufferLimit())
                             // Writing threads need another segment now.
                             continue;

                         // Writing threads are not waiting for new segments, we can spend time on other tasks.
                         // flush old Cfs if we're full
                         maybeFlushToReclaim();
                     }
                     catch (Throwable t)
                     {
                         JVMStabilityInspector.inspectThrowable(t);
                         if (!CommitLog.handleCommitError("Failed managing commit log segments", t))
                             return;
                         // sleep some arbitrary period to avoid spamming CL
                         Uninterruptibles.sleepUninterruptibly(1, TimeUnit.SECONDS);

                         // If we offered a segment, wait for it to be taken before reentering the loop.
                         // There could be a new segment in next not offered, but only on failure to discard it while
                         // shutting down-- nothing more can or needs to be done in that case.
                     }

                     WaitQueue.waitOnCondition(managerThreadWaitCondition, managerThreadWaitQueue);
                 }
             }
         };

         shutdown = false;
         managerThread = NamedThreadFactory.createThread(runnable, "COMMIT-LOG-ALLOCATOR");
         managerThread.start();

         // for simplicity, ensure the first segment is allocated before continuing
         advanceAllocatingFrom(null);
     }

     private boolean atSegmentBufferLimit()
     {
         return CommitLogSegment.usesBufferPool(commitLog) && bufferPool.atLimit();
     }

     private void maybeFlushToReclaim()
     {
         long unused = unusedCapacity();
         if (unused < 0)
         {
             long flushingSize = 0;
             List<CommitLogSegment> segmentsToRecycle = new ArrayList<>();
             for (CommitLogSegment segment : activeSegments)
             {
                 if (segment == allocatingFrom)
                     break;
                 flushingSize += segment.onDiskSize();
                 segmentsToRecycle.add(segment);
                 if (flushingSize + unused >= 0)
                     break;
             }
             flushDataFrom(segmentsToRecycle, false);
         }
     }


     /**
      * Allocate a segment within this CLSM. Should either succeed or throw.
      */
     public abstract Allocation allocate(Mutation mutation, int size);

     /**
      * The recovery and replay process replays mutations into memtables and flushes them to disk. Individual CLSM
      * decide what to do with those segments on disk after they've been replayed.
      */
     abstract void handleReplayedSegment(final File file);

     /**
      * Hook to allow segment managers to track state surrounding creation of new segments. Onl perform as task submit
      * to segment manager so it's performed on segment management thread.
      */
     abstract CommitLogSegment createSegment();

     /**
      * Indicates that a segment file has been flushed and is no longer needed. Only perform as task submit to segment
      * manager so it's performend on segment management thread, or perform while segment management thread is shutdown
      * during testing resets.
      *
      * @param segment segment to be discarded
      * @param delete  whether or not the segment is safe to be deleted.
      */
     abstract void discard(CommitLogSegment segment, boolean delete);

     /**
      * Advances the allocatingFrom pointer to the next prepared segment, but only if it is currently the segment provided.
      *
      * WARNING: Assumes segment management thread always succeeds in allocating a new segment or kills the JVM.
      */
     @DontInline
     void advanceAllocatingFrom(CommitLogSegment old)
     {
         while (true)
         {
             synchronized (this)
             {
                 // do this in a critical section so we can maintain the order of segment construction when moving to allocatingFrom/activeSegments
                 if (allocatingFrom != old)
                     return;

                 // If a segment is ready, take it now, otherwise wait for the management thread to construct it.
                 if (availableSegment != null)
                 {
                     // Success! Change allocatingFrom and activeSegments (which must be kept in order) before leaving
                     // the critical section.
                     activeSegments.add(allocatingFrom = availableSegment);
                     availableSegment = null;
                     break;
                 }
             }

             awaitAvailableSegment(old);
         }

         // Signal the management thread to prepare a new segment.
         wakeManager();

         if (old != null)
         {
             // Now we can run the user defined command just after switching to the new commit log.
             // (Do this here instead of in the recycle call so we can get a head start on the archive.)
             commitLog.archiver.maybeArchive(old);

             // ensure we don't continue to use the old file; not strictly necessary, but cleaner to enforce it
             old.discardUnusedTail();
         }

         // request that the CL be synced out-of-band, as we've finished a segment
         commitLog.requestExtraSync();
     }

     void awaitAvailableSegment(CommitLogSegment currentAllocatingFrom)
     {
         do
         {
             WaitQueue.Signal prepared = segmentPrepared.register(commitLog.metrics.waitingOnSegmentAllocation.time());
             if (availableSegment == null && allocatingFrom == currentAllocatingFrom)
                 prepared.awaitUninterruptibly();
             else
                 prepared.cancel();
         }
         while (availableSegment == null && allocatingFrom == currentAllocatingFrom);
     }

     /**
      * Switch to a new segment, regardless of how much is left in the current one.
      *
      * Flushes any dirty CFs for this segment and any older segments, and then discards the segments
      */
     void forceRecycleAll(Iterable<UUID> droppedCfs)
     {
         List<CommitLogSegment> segmentsToRecycle = new ArrayList<>(activeSegments);
         CommitLogSegment last = segmentsToRecycle.get(segmentsToRecycle.size() - 1);
         advanceAllocatingFrom(last);

         // wait for the commit log modifications
         last.waitForModifications();

         // make sure the writes have materialized inside of the memtables by waiting for all outstanding writes
         // to complete
         Keyspace.writeOrder.awaitNewBarrier();

         // flush and wait for all CFs that are dirty in segments up-to and including 'last'
         Future<?> future = flushDataFrom(segmentsToRecycle, true);
         try
         {
             future.get();

             for (CommitLogSegment segment : activeSegments)
                 for (UUID cfId : droppedCfs)
                     segment.markClean(cfId, CommitLogPosition.NONE, segment.getCurrentCommitLogPosition());

             // now recycle segments that are unused, as we may not have triggered a discardCompletedSegments()
             // if the previous active segment was the only one to recycle (since an active segment isn't
             // necessarily dirty, and we only call dCS after a flush).
             for (CommitLogSegment segment : activeSegments)
             {
                 if (segment.isUnused())
                     archiveAndDiscard(segment);
             }

             CommitLogSegment first;
             if ((first = activeSegments.peek()) != null && first.id <= last.id)
                 logger.error("Failed to force-recycle all segments; at least one segment is still in use with dirty CFs.");
         }
         catch (Throwable t)
         {
             // for now just log the error
             logger.error("Failed waiting for a forced recycle of in-use commit log segments", t);
         }
     }

     /**
      * Indicates that a segment is no longer in use and that it should be discarded.
      *
      * @param segment segment that is no longer in use
      */
     void archiveAndDiscard(final CommitLogSegment segment)
     {
         boolean archiveSuccess = commitLog.archiver.maybeWaitForArchiving(segment.getName());
         if (!activeSegments.remove(segment))
             return; // already discarded
         // if archiving (command) was not successful then leave the file alone. don't delete or recycle.
         logger.debug("Segment {} is no longer active and will be deleted {}", segment, archiveSuccess ? "now" : "by the archive script");
         discard(segment, archiveSuccess);
     }

     /**
      * Adjust the tracked on-disk size. Called by individual segments to reflect writes, allocations and discards.
      * @param addedSize
      */
     void addSize(long addedSize)
     {
         size.addAndGet(addedSize);
     }

     /**
      * @return the space (in bytes) used by all segment files.
      */
     public long onDiskSize()
     {
         return size.get();
     }

     private long unusedCapacity()
     {
         long total = DatabaseDescriptor.getTotalCommitlogSpaceInMB() * 1024 * 1024;
         long currentSize = size.get();
         logger.trace("Total active commitlog segment space used is {} out of {}", currentSize, total);
         return total - currentSize;
     }

     /**
      * Force a flush on all CFs that are still dirty in @param segments.
      *
      * @return a Future that will finish when all the flushes are complete.
      */
     private Future<?> flushDataFrom(List<CommitLogSegment> segments, boolean force)
     {
         if (segments.isEmpty())
             return Futures.immediateFuture(null);
         final CommitLogPosition maxCommitLogPosition = segments.get(segments.size() - 1).getCurrentCommitLogPosition();

         // a map of CfId -> forceFlush() to ensure we only queue one flush per cf
         final Map<UUID, ListenableFuture<?>> flushes = new LinkedHashMap<>();

         for (CommitLogSegment segment : segments)
         {
             for (UUID dirtyCFId : segment.getDirtyCFIDs())
             {
                 Pair<String,String> pair = Schema.instance.getCF(dirtyCFId);
                 if (pair == null)
                 {
                     // even though we remove the schema entry before a final flush when dropping a CF,
                     // it's still possible for a writer to race and finish his append after the flush.
                     logger.trace("Marking clean CF {} that doesn't exist anymore", dirtyCFId);
                     segment.markClean(dirtyCFId, CommitLogPosition.NONE, segment.getCurrentCommitLogPosition());
                 }
                 else if (!flushes.containsKey(dirtyCFId))
                 {
                     String keyspace = pair.left;
                     final ColumnFamilyStore cfs = Keyspace.open(keyspace).getColumnFamilyStore(dirtyCFId);
                     // can safely call forceFlush here as we will only ever block (briefly) for other attempts to flush,
                     // no deadlock possibility since switchLock removal
                     flushes.put(dirtyCFId, force ? cfs.forceFlush() : cfs.forceFlush(maxCommitLogPosition));
                 }
             }
         }

         return Futures.allAsList(flushes.values());
     }

     /**
      * Stops CL, for testing purposes. DO NOT USE THIS OUTSIDE OF TESTS.
      * Only call this after the AbstractCommitLogService is shut down.
      */
     public void stopUnsafe(boolean deleteSegments)
     {
         logger.debug("CLSM closing and clearing existing commit log segments...");

         shutdown();
         try
         {
             awaitTermination();
         }
         catch (InterruptedException e)
         {
             throw new RuntimeException(e);
         }

         for (CommitLogSegment segment : activeSegments)
             closeAndDeleteSegmentUnsafe(segment, deleteSegments);
         activeSegments.clear();

         size.set(0L);

         logger.trace("CLSM done with closing and clearing existing commit log segments.");
     }

     /**
      * To be used by tests only. Not safe if mutation slots are being allocated concurrently.
      */
     void awaitManagementTasksCompletion()
     {
         if (availableSegment == null && !atSegmentBufferLimit())
         {
             awaitAvailableSegment(allocatingFrom);
         }
     }

     /**
      * Explicitly for use only during resets in unit testing.
      */
     private void closeAndDeleteSegmentUnsafe(CommitLogSegment segment, boolean delete)
     {
         try
         {
             discard(segment, delete);
         }
         catch (AssertionError ignored)
         {
             // segment file does not exist
         }
     }

     /**
      * Initiates the shutdown process for the management thread.
      */
     public void shutdown()
     {
         assert !shutdown;
         shutdown = true;

         // Release the management thread and delete prepared segment.
         // Do not block as another thread may claim the segment (this can happen during unit test initialization).
         discardAvailableSegment();
         wakeManager();
     }

     private void discardAvailableSegment()
     {
         CommitLogSegment next = null;
         synchronized (this)
         {
             next = availableSegment;
             availableSegment = null;
         }
         if (next != null)
             next.discard(true);
     }

     /**
      * Returns when the management thread terminates.
      */
     public void awaitTermination() throws InterruptedException
     {
         managerThread.join();
         managerThread = null;

         for (CommitLogSegment segment : activeSegments)
             segment.close();

         bufferPool.shutdown();
     }

     /**
      * @return a read-only collection of the active commit log segments
      */
     @VisibleForTesting
     public Collection<CommitLogSegment> getActiveSegments()
     {
         return Collections.unmodifiableCollection(activeSegments);
     }

     /**
      * @return the current CommitLogPosition of the active segment we're allocating from
      */
     CommitLogPosition getCurrentPosition()
     {
         return allocatingFrom.getCurrentCommitLogPosition();
     }

     /**
      * Requests commit log files sync themselves, if needed. This may or may not involve flushing to disk.
      *
      * @param flush Request that the sync operation flush the file to disk.
      */
     public void sync(boolean flush) throws IOException
     {
         CommitLogSegment current = allocatingFrom;
         for (CommitLogSegment segment : getActiveSegments())
         {
             // Do not sync segments that became active after sync started.
             if (segment.id > current.id)
                 return;
             segment.sync(flush);
         }
     }

     /**
      * Used by compressed and encrypted segments to share a buffer pool across the CLSM.
      */
     SimpleCachedBufferPool getBufferPool()
     {
         return bufferPool;
     }

     void wakeManager()
     {
         managerThreadWaitQueue.signalAll();
     }

     /**
      * Called by commit log segments when a buffer is freed to wake the management thread, which may be waiting for
      * a buffer to become available.
      */
     void notifyBufferFreed()
     {
         wakeManager();
     }

     /** Read-only access to current segment for subclasses. */
     CommitLogSegment allocatingFrom()
     {
         return allocatingFrom;
     }
 }
	/*
	* Licensed to the Apache Software Foundation (ASF) under one
	* or more contributor license agreements. See the NOTICE file
	* distributed with this work for additional information
	* regarding copyright ownership. The ASF licenses this file
	* to you under the Apache License, Version 2.0 (the
	* "License"); you may not use this file except in compliance
	* with the License. You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/
	package org.apache.cassandra.db.commitlog;

	import java.io.File;
	import java.io.IOException;
	import java.util.*;
	import java.util.concurrent.*;
	import java.util.concurrent.atomic.AtomicLong;
	import java.util.function.BooleanSupplier;

	import com.google.common.annotations.VisibleForTesting;
	import com.google.common.util.concurrent.*;
	import org.slf4j.Logger;
	import org.slf4j.LoggerFactory;

	import net.nicoulaj.compilecommand.annotations.DontInline;
	import org.apache.cassandra.concurrent.NamedThreadFactory;
	import org.apache.cassandra.config.DatabaseDescriptor;
	import org.apache.cassandra.config.Schema;
	import org.apache.cassandra.db.*;
	import org.apache.cassandra.utils.*;
	import org.apache.cassandra.utils.concurrent.WaitQueue;

	import static org.apache.cassandra.db.commitlog.CommitLogSegment.Allocation;

	/**
	* Performs eager-creation of commit log segments in a background thread. All the
	* public methods are thread safe.
	*/
	public abstract class AbstractCommitLogSegmentManager
	{
	static final Logger logger = LoggerFactory.getLogger(AbstractCommitLogSegmentManager.class);

	/**
	* Segment that is ready to be used. The management thread fills this and blocks until consumed.
	*
	* A single management thread produces this, and consumers are already synchronizing to make sure other work is
	* performed atomically with consuming this. Volatile to make sure writes by the management thread become
	* visible (ordered/lazySet would suffice). Consumers (advanceAllocatingFrom and discardAvailableSegment) must
	* synchronize on 'this'.
	*/
	private volatile CommitLogSegment availableSegment = null;

	private final WaitQueue segmentPrepared = new WaitQueue();

	/** Active segments, containing unflushed data. The tail of this queue is the one we allocate writes to */
	private final ConcurrentLinkedQueue<CommitLogSegment> activeSegments = new ConcurrentLinkedQueue<>();

	/**
	* The segment we are currently allocating commit log records to.
	*
	* Written by advanceAllocatingFrom which synchronizes on 'this'. Volatile to ensure reads get current value.
	*/
	private volatile CommitLogSegment allocatingFrom = null;

	final String storageDirectory;

	/**
	* Tracks commitlog size, in multiples of the segment size. We need to do this so we can "promise" size
	* adjustments ahead of actually adding/freeing segments on disk, so that the "evict oldest segment" logic
	* can see the effect of recycling segments immediately (even though they're really happening asynchronously
	* on the manager thread, which will take a ms or two).
	*/
	private final AtomicLong size = new AtomicLong();

	private Thread managerThread;
	protected final CommitLog commitLog;
	private volatile boolean shutdown;
	private final BooleanSupplier managerThreadWaitCondition = () -> (availableSegment == null && !atSegmentBufferLimit()) \|\| shutdown;
	private final WaitQueue managerThreadWaitQueue = new WaitQueue();

	private static final SimpleCachedBufferPool bufferPool =
	new SimpleCachedBufferPool(DatabaseDescriptor.getCommitLogMaxCompressionBuffersInPool(), DatabaseDescriptor.getCommitLogSegmentSize());

	AbstractCommitLogSegmentManager(final CommitLog commitLog, String storageDirectory)
	{
	this.commitLog = commitLog;
	this.storageDirectory = storageDirectory;
	}

	void start()
	{
	// The run loop for the manager thread
	Runnable runnable = new WrappedRunnable()
	{
	public void runMayThrow() throws Exception
	{
	while (!shutdown)
	{
	try
	{
	assert availableSegment == null;
	logger.trace("No segments in reserve; creating a fresh one");
	availableSegment = createSegment();
	if (shutdown)
	{
	// If shutdown() started and finished during segment creation, we are now left with a
	// segment that no one will consume. Discard it.
	discardAvailableSegment();
	return;
	}

	segmentPrepared.signalAll();
	Thread.yield();

	if (availableSegment == null && !atSegmentBufferLimit())
	// Writing threads need another segment now.
	continue;

	// Writing threads are not waiting for new segments, we can spend time on other tasks.
	// flush old Cfs if we're full
	maybeFlushToReclaim();
	}
	catch (Throwable t)
	{
	JVMStabilityInspector.inspectThrowable(t);
	if (!CommitLog.handleCommitError("Failed managing commit log segments", t))
	return;
	// sleep some arbitrary period to avoid spamming CL
	Uninterruptibles.sleepUninterruptibly(1, TimeUnit.SECONDS);

	// If we offered a segment, wait for it to be taken before reentering the loop.
	// There could be a new segment in next not offered, but only on failure to discard it while
	// shutting down-- nothing more can or needs to be done in that case.
	}

	WaitQueue.waitOnCondition(managerThreadWaitCondition, managerThreadWaitQueue);
	}
	}
	};

	shutdown = false;
	managerThread = NamedThreadFactory.createThread(runnable, "COMMIT-LOG-ALLOCATOR");
	managerThread.start();

	// for simplicity, ensure the first segment is allocated before continuing
	advanceAllocatingFrom(null);
	}

	private boolean atSegmentBufferLimit()
	{
	return CommitLogSegment.usesBufferPool(commitLog) && bufferPool.atLimit();
	}

	private void maybeFlushToReclaim()
	{
	long unused = unusedCapacity();
	if (unused < 0)
	{
	long flushingSize = 0;
	List<CommitLogSegment> segmentsToRecycle = new ArrayList<>();
	for (CommitLogSegment segment : activeSegments)
	{
	if (segment == allocatingFrom)
	break;
	flushingSize += segment.onDiskSize();
	segmentsToRecycle.add(segment);
	if (flushingSize + unused >= 0)
	break;
	}
	flushDataFrom(segmentsToRecycle, false);
	}
	}


	/**
	* Allocate a segment within this CLSM. Should either succeed or throw.
	*/
	public abstract Allocation allocate(Mutation mutation, int size);

	/**
	* The recovery and replay process replays mutations into memtables and flushes them to disk. Individual CLSM
	* decide what to do with those segments on disk after they've been replayed.
	*/
	abstract void handleReplayedSegment(final File file);

	/**
	* Hook to allow segment managers to track state surrounding creation of new segments. Onl perform as task submit
	* to segment manager so it's performed on segment management thread.
	*/
	abstract CommitLogSegment createSegment();

	/**
	* Indicates that a segment file has been flushed and is no longer needed. Only perform as task submit to segment
	* manager so it's performend on segment management thread, or perform while segment management thread is shutdown
	* during testing resets.
	*
	* @param segment segment to be discarded
	* @param delete whether or not the segment is safe to be deleted.
	*/
	abstract void discard(CommitLogSegment segment, boolean delete);

	/**
	* Advances the allocatingFrom pointer to the next prepared segment, but only if it is currently the segment provided.
	*
	* WARNING: Assumes segment management thread always succeeds in allocating a new segment or kills the JVM.
	*/
	@DontInline
	void advanceAllocatingFrom(CommitLogSegment old)
	{
	while (true)
	{
	synchronized (this)
	{
	// do this in a critical section so we can maintain the order of segment construction when moving to allocatingFrom/activeSegments
	if (allocatingFrom != old)
	return;

	// If a segment is ready, take it now, otherwise wait for the management thread to construct it.
	if (availableSegment != null)
	{
	// Success! Change allocatingFrom and activeSegments (which must be kept in order) before leaving
	// the critical section.
	activeSegments.add(allocatingFrom = availableSegment);
	availableSegment = null;
	break;
	}
	}

	awaitAvailableSegment(old);
	}

	// Signal the management thread to prepare a new segment.
	wakeManager();

	if (old != null)
	{
	// Now we can run the user defined command just after switching to the new commit log.
	// (Do this here instead of in the recycle call so we can get a head start on the archive.)
	commitLog.archiver.maybeArchive(old);

	// ensure we don't continue to use the old file; not strictly necessary, but cleaner to enforce it
	old.discardUnusedTail();
	}

	// request that the CL be synced out-of-band, as we've finished a segment
	commitLog.requestExtraSync();
	}

	void awaitAvailableSegment(CommitLogSegment currentAllocatingFrom)
	{
	do
	{
	WaitQueue.Signal prepared = segmentPrepared.register(commitLog.metrics.waitingOnSegmentAllocation.time());
	if (availableSegment == null && allocatingFrom == currentAllocatingFrom)
	prepared.awaitUninterruptibly();
	else
	prepared.cancel();
	}
	while (availableSegment == null && allocatingFrom == currentAllocatingFrom);
	}

	/**
	* Switch to a new segment, regardless of how much is left in the current one.
	*
	* Flushes any dirty CFs for this segment and any older segments, and then discards the segments
	*/
	void forceRecycleAll(Iterable<UUID> droppedCfs)
	{
	List<CommitLogSegment> segmentsToRecycle = new ArrayList<>(activeSegments);
	CommitLogSegment last = segmentsToRecycle.get(segmentsToRecycle.size() - 1);
	advanceAllocatingFrom(last);

	// wait for the commit log modifications
	last.waitForModifications();

	// make sure the writes have materialized inside of the memtables by waiting for all outstanding writes
	// to complete
	Keyspace.writeOrder.awaitNewBarrier();

	// flush and wait for all CFs that are dirty in segments up-to and including 'last'
	Future<?> future = flushDataFrom(segmentsToRecycle, true);
	try
	{
	future.get();

	for (CommitLogSegment segment : activeSegments)
	for (UUID cfId : droppedCfs)
	segment.markClean(cfId, CommitLogPosition.NONE, segment.getCurrentCommitLogPosition());

	// now recycle segments that are unused, as we may not have triggered a discardCompletedSegments()
	// if the previous active segment was the only one to recycle (since an active segment isn't
	// necessarily dirty, and we only call dCS after a flush).
	for (CommitLogSegment segment : activeSegments)
	{
	if (segment.isUnused())
	archiveAndDiscard(segment);
	}

	CommitLogSegment first;
	if ((first = activeSegments.peek()) != null && first.id <= last.id)
	logger.error("Failed to force-recycle all segments; at least one segment is still in use with dirty CFs.");
	}
	catch (Throwable t)
	{
	// for now just log the error
	logger.error("Failed waiting for a forced recycle of in-use commit log segments", t);
	}
	}

	/**
	* Indicates that a segment is no longer in use and that it should be discarded.
	*
	* @param segment segment that is no longer in use
	*/
	void archiveAndDiscard(final CommitLogSegment segment)
	{
	boolean archiveSuccess = commitLog.archiver.maybeWaitForArchiving(segment.getName());
	if (!activeSegments.remove(segment))
	return; // already discarded
	// if archiving (command) was not successful then leave the file alone. don't delete or recycle.
	logger.debug("Segment {} is no longer active and will be deleted {}", segment, archiveSuccess ? "now" : "by the archive script");
	discard(segment, archiveSuccess);
	}

	/**
	* Adjust the tracked on-disk size. Called by individual segments to reflect writes, allocations and discards.
	* @param addedSize
	*/
	void addSize(long addedSize)
	{
	size.addAndGet(addedSize);
	}

	/**
	* @return the space (in bytes) used by all segment files.
	*/
	public long onDiskSize()
	{
	return size.get();
	}

	private long unusedCapacity()
	{
	long total = DatabaseDescriptor.getTotalCommitlogSpaceInMB() * 1024 * 1024;
	long currentSize = size.get();
	logger.trace("Total active commitlog segment space used is {} out of {}", currentSize, total);
	return total - currentSize;
	}

	/**
	* Force a flush on all CFs that are still dirty in @param segments.
	*
	* @return a Future that will finish when all the flushes are complete.
	*/
	private Future<?> flushDataFrom(List<CommitLogSegment> segments, boolean force)
	{
	if (segments.isEmpty())
	return Futures.immediateFuture(null);
	final CommitLogPosition maxCommitLogPosition = segments.get(segments.size() - 1).getCurrentCommitLogPosition();

	// a map of CfId -> forceFlush() to ensure we only queue one flush per cf
	final Map<UUID, ListenableFuture<?>> flushes = new LinkedHashMap<>();

	for (CommitLogSegment segment : segments)
	{
	for (UUID dirtyCFId : segment.getDirtyCFIDs())
	{
	Pair<String,String> pair = Schema.instance.getCF(dirtyCFId);
	if (pair == null)
	{
	// even though we remove the schema entry before a final flush when dropping a CF,
	// it's still possible for a writer to race and finish his append after the flush.
	logger.trace("Marking clean CF {} that doesn't exist anymore", dirtyCFId);
	segment.markClean(dirtyCFId, CommitLogPosition.NONE, segment.getCurrentCommitLogPosition());
	}
	else if (!flushes.containsKey(dirtyCFId))
	{
	String keyspace = pair.left;
	final ColumnFamilyStore cfs = Keyspace.open(keyspace).getColumnFamilyStore(dirtyCFId);
	// can safely call forceFlush here as we will only ever block (briefly) for other attempts to flush,
	// no deadlock possibility since switchLock removal
	flushes.put(dirtyCFId, force ? cfs.forceFlush() : cfs.forceFlush(maxCommitLogPosition));
	}
	}
	}

	return Futures.allAsList(flushes.values());
	}

	/**
	* Stops CL, for testing purposes. DO NOT USE THIS OUTSIDE OF TESTS.
	* Only call this after the AbstractCommitLogService is shut down.
	*/
	public void stopUnsafe(boolean deleteSegments)
	{
	logger.debug("CLSM closing and clearing existing commit log segments...");

	shutdown();
	try
	{
	awaitTermination();
	}
	catch (InterruptedException e)
	{
	throw new RuntimeException(e);
	}

	for (CommitLogSegment segment : activeSegments)
	closeAndDeleteSegmentUnsafe(segment, deleteSegments);
	activeSegments.clear();

	size.set(0L);

	logger.trace("CLSM done with closing and clearing existing commit log segments.");
	}

	/**
	* To be used by tests only. Not safe if mutation slots are being allocated concurrently.
	*/
	void awaitManagementTasksCompletion()
	{
	if (availableSegment == null && !atSegmentBufferLimit())
	{
	awaitAvailableSegment(allocatingFrom);
	}
	}

	/**
	* Explicitly for use only during resets in unit testing.
	*/
	private void closeAndDeleteSegmentUnsafe(CommitLogSegment segment, boolean delete)
	{
	try
	{
	discard(segment, delete);
	}
	catch (AssertionError ignored)
	{
	// segment file does not exist
	}
	}

	/**
	* Initiates the shutdown process for the management thread.
	*/
	public void shutdown()
	{
	assert !shutdown;
	shutdown = true;

	// Release the management thread and delete prepared segment.
	// Do not block as another thread may claim the segment (this can happen during unit test initialization).
	discardAvailableSegment();
	wakeManager();
	}

	private void discardAvailableSegment()
	{
	CommitLogSegment next = null;
	synchronized (this)
	{
	next = availableSegment;
	availableSegment = null;
	}
	if (next != null)
	next.discard(true);
	}

	/**
	* Returns when the management thread terminates.
	*/
	public void awaitTermination() throws InterruptedException
	{
	managerThread.join();
	managerThread = null;

	for (CommitLogSegment segment : activeSegments)
	segment.close();

	bufferPool.shutdown();
	}

	/**
	* @return a read-only collection of the active commit log segments
	*/
	@VisibleForTesting
	public Collection<CommitLogSegment> getActiveSegments()
	{
	return Collections.unmodifiableCollection(activeSegments);
	}

	/**
	* @return the current CommitLogPosition of the active segment we're allocating from
	*/
	CommitLogPosition getCurrentPosition()
	{
	return allocatingFrom.getCurrentCommitLogPosition();
	}

	/**
	* Requests commit log files sync themselves, if needed. This may or may not involve flushing to disk.
	*
	* @param flush Request that the sync operation flush the file to disk.
	*/
	public void sync(boolean flush) throws IOException
	{
	CommitLogSegment current = allocatingFrom;
	for (CommitLogSegment segment : getActiveSegments())
	{
	// Do not sync segments that became active after sync started.
	if (segment.id > current.id)
	return;
	segment.sync(flush);
	}
	}

	/**
	* Used by compressed and encrypted segments to share a buffer pool across the CLSM.
	*/
	SimpleCachedBufferPool getBufferPool()
	{
	return bufferPool;
	}

	void wakeManager()
	{
	managerThreadWaitQueue.signalAll();
	}

	/**
	* Called by commit log segments when a buffer is freed to wake the management thread, which may be waiting for
	* a buffer to become available.
	*/
	void notifyBufferFreed()
	{
	wakeManager();
	}

	/** Read-only access to current segment for subclasses. */
	CommitLogSegment allocatingFrom()
	{
	return allocatingFrom;
	}
	}