001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    package org.apache.hadoop.hdfs.server.datanode.fsdataset;
019    
020    import java.io.IOException;
021    import java.util.List;
022    
023    import org.apache.hadoop.util.DiskChecker.DiskOutOfSpaceException;
024    
025    /**
026     * Choose volumes in round-robin order.
027     */
028    public class RoundRobinVolumeChoosingPolicy<V extends FsVolumeSpi>
029        implements VolumeChoosingPolicy<V> {
030    
031      private int curVolume = 0;
032    
033      @Override
034      public synchronized V chooseVolume(final List<V> volumes, final long blockSize
035          ) throws IOException {
036        if(volumes.size() < 1) {
037          throw new DiskOutOfSpaceException("No more available volumes");
038        }
039        
040        // since volumes could've been removed because of the failure
041        // make sure we are not out of bounds
042        if(curVolume >= volumes.size()) {
043          curVolume = 0;
044        }
045        
046        int startVolume = curVolume;
047        long maxAvailable = 0;
048        
049        while (true) {
050          final V volume = volumes.get(curVolume);
051          curVolume = (curVolume + 1) % volumes.size();
052          long availableVolumeSize = volume.getAvailable();
053          if (availableVolumeSize > blockSize) { return volume; }
054          
055          if (availableVolumeSize > maxAvailable) {
056            maxAvailable = availableVolumeSize;
057          }
058          
059          if (curVolume == startVolume) {
060            throw new DiskOutOfSpaceException("Out of space: "
061                + "The volume with the most available space (=" + maxAvailable
062                + " B) is less than the block size (=" + blockSize + " B).");
063          }
064        }
065      }
066    }