HDFS-6955. DN should reserve disk space for a full block when creating tmp files (Contributed by Kanaka Kumar Avvaru)

This commit is contained in:
Vinayakumar B 2015-09-18 16:42:22 +05:30
parent 22dee29857
commit f40b4f4542
8 changed files with 198 additions and 56 deletions

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@ -117,7 +117,7 @@ class BlockReceiver implements Closeable {
/** the block to receive */ /** the block to receive */
private final ExtendedBlock block; private final ExtendedBlock block;
/** the replica to write */ /** the replica to write */
private final ReplicaInPipelineInterface replicaInfo; private ReplicaInPipelineInterface replicaInfo;
/** pipeline stage */ /** pipeline stage */
private final BlockConstructionStage stage; private final BlockConstructionStage stage;
private final boolean isTransfer; private final boolean isTransfer;
@ -259,6 +259,9 @@ class BlockReceiver implements Closeable {
} catch (ReplicaNotFoundException bne) { } catch (ReplicaNotFoundException bne) {
throw bne; throw bne;
} catch(IOException ioe) { } catch(IOException ioe) {
if (replicaInfo != null) {
replicaInfo.releaseAllBytesReserved();
}
IOUtils.closeStream(this); IOUtils.closeStream(this);
cleanupBlock(); cleanupBlock();

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@ -62,13 +62,13 @@ public interface FsVolumeSpi {
boolean isTransientStorage(); boolean isTransientStorage();
/** /**
* Reserve disk space for an RBW block so a writer does not run out of * Reserve disk space for a block (RBW or Re-replicating)
* space before the block is full. * so a writer does not run out of space before the block is full.
*/ */
void reserveSpaceForRbw(long bytesToReserve); void reserveSpaceForReplica(long bytesToReserve);
/** /**
* Release disk space previously reserved for RBW block. * Release disk space previously reserved for block opened for write.
*/ */
void releaseReservedSpace(long bytesToRelease); void releaseReservedSpace(long bytesToRelease);

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@ -1162,7 +1162,7 @@ class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
// Replace finalized replica by a RBW replica in replicas map // Replace finalized replica by a RBW replica in replicas map
volumeMap.add(bpid, newReplicaInfo); volumeMap.add(bpid, newReplicaInfo);
v.reserveSpaceForRbw(estimateBlockLen - replicaInfo.getNumBytes()); v.reserveSpaceForReplica(estimateBlockLen - replicaInfo.getNumBytes());
return newReplicaInfo; return newReplicaInfo;
} }
@ -1492,7 +1492,7 @@ class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
} }
ReplicaInPipeline newReplicaInfo = ReplicaInPipeline newReplicaInfo =
new ReplicaInPipeline(b.getBlockId(), b.getGenerationStamp(), v, new ReplicaInPipeline(b.getBlockId(), b.getGenerationStamp(), v,
f.getParentFile(), 0); f.getParentFile(), b.getLocalBlock().getNumBytes());
volumeMap.add(b.getBlockPoolId(), newReplicaInfo); volumeMap.add(b.getBlockPoolId(), newReplicaInfo);
return new ReplicaHandler(newReplicaInfo, ref); return new ReplicaHandler(newReplicaInfo, ref);
} else { } else {
@ -2559,14 +2559,15 @@ class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
final long usedSpace; // size of space used by HDFS final long usedSpace; // size of space used by HDFS
final long freeSpace; // size of free space excluding reserved space final long freeSpace; // size of free space excluding reserved space
final long reservedSpace; // size of space reserved for non-HDFS final long reservedSpace; // size of space reserved for non-HDFS
final long reservedSpaceForRBW; // size of space reserved RBW final long reservedSpaceForReplicas; // size of space reserved RBW or
// re-replication
VolumeInfo(FsVolumeImpl v, long usedSpace, long freeSpace) { VolumeInfo(FsVolumeImpl v, long usedSpace, long freeSpace) {
this.directory = v.toString(); this.directory = v.toString();
this.usedSpace = usedSpace; this.usedSpace = usedSpace;
this.freeSpace = freeSpace; this.freeSpace = freeSpace;
this.reservedSpace = v.getReserved(); this.reservedSpace = v.getReserved();
this.reservedSpaceForRBW = v.getReservedForRbw(); this.reservedSpaceForReplicas = v.getReservedForReplicas();
} }
} }
@ -2600,7 +2601,7 @@ class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
innerInfo.put("usedSpace", v.usedSpace); innerInfo.put("usedSpace", v.usedSpace);
innerInfo.put("freeSpace", v.freeSpace); innerInfo.put("freeSpace", v.freeSpace);
innerInfo.put("reservedSpace", v.reservedSpace); innerInfo.put("reservedSpace", v.reservedSpace);
innerInfo.put("reservedSpaceForRBW", v.reservedSpaceForRBW); innerInfo.put("reservedSpaceForReplicas", v.reservedSpaceForReplicas);
info.put(v.directory, innerInfo); info.put(v.directory, innerInfo);
} }
return info; return info;

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@ -22,8 +22,8 @@ import java.io.File;
import java.io.FileOutputStream; import java.io.FileOutputStream;
import java.io.FilenameFilter; import java.io.FilenameFilter;
import java.io.IOException; import java.io.IOException;
import java.nio.channels.ClosedChannelException;
import java.io.OutputStreamWriter; import java.io.OutputStreamWriter;
import java.nio.channels.ClosedChannelException;
import java.nio.file.Files; import java.nio.file.Files;
import java.nio.file.Paths; import java.nio.file.Paths;
import java.nio.file.StandardCopyOption; import java.nio.file.StandardCopyOption;
@ -40,9 +40,6 @@ import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong; import java.util.concurrent.atomic.AtomicLong;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Joiner;
import com.google.common.base.Preconditions;
import org.apache.hadoop.classification.InterfaceAudience; import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.DF; import org.apache.hadoop.fs.DF;
@ -54,21 +51,24 @@ import org.apache.hadoop.hdfs.protocol.BlockListAsLongs;
import org.apache.hadoop.hdfs.protocol.ExtendedBlock; import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
import org.apache.hadoop.hdfs.server.datanode.DataStorage; import org.apache.hadoop.hdfs.server.datanode.DataStorage;
import org.apache.hadoop.hdfs.server.datanode.DatanodeUtil; import org.apache.hadoop.hdfs.server.datanode.DatanodeUtil;
import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsVolumeReference;
import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsDatasetSpi; import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsDatasetSpi;
import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsVolumeReference;
import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsVolumeSpi; import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsVolumeSpi;
import org.apache.hadoop.hdfs.server.protocol.DatanodeStorage; import org.apache.hadoop.hdfs.server.protocol.DatanodeStorage;
import org.apache.hadoop.util.CloseableReferenceCount;
import org.apache.hadoop.io.IOUtils; import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.util.CloseableReferenceCount;
import org.apache.hadoop.util.DiskChecker.DiskErrorException; import org.apache.hadoop.util.DiskChecker.DiskErrorException;
import com.google.common.util.concurrent.ThreadFactoryBuilder;
import org.apache.hadoop.util.Time; import org.apache.hadoop.util.Time;
import org.codehaus.jackson.annotate.JsonProperty; import org.codehaus.jackson.annotate.JsonProperty;
import org.codehaus.jackson.map.ObjectMapper; import org.codehaus.jackson.map.ObjectMapper;
import org.slf4j.Logger; import org.slf4j.Logger;
import org.slf4j.LoggerFactory; import org.slf4j.LoggerFactory;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Joiner;
import com.google.common.base.Preconditions;
import com.google.common.util.concurrent.ThreadFactoryBuilder;
/** /**
* The underlying volume used to store replica. * The underlying volume used to store replica.
* *
@ -90,8 +90,9 @@ public class FsVolumeImpl implements FsVolumeSpi {
private final long reserved; private final long reserved;
private CloseableReferenceCount reference = new CloseableReferenceCount(); private CloseableReferenceCount reference = new CloseableReferenceCount();
// Disk space reserved for open blocks. // Disk space reserved for blocks (RBW or Re-replicating) open for write.
private AtomicLong reservedForRbw; private AtomicLong reservedForReplicas;
private long recentReserved = 0;
// Capacity configured. This is useful when we want to // Capacity configured. This is useful when we want to
// limit the visible capacity for tests. If negative, then we just // limit the visible capacity for tests. If negative, then we just
@ -113,7 +114,7 @@ public class FsVolumeImpl implements FsVolumeSpi {
this.reserved = conf.getLong( this.reserved = conf.getLong(
DFSConfigKeys.DFS_DATANODE_DU_RESERVED_KEY, DFSConfigKeys.DFS_DATANODE_DU_RESERVED_KEY,
DFSConfigKeys.DFS_DATANODE_DU_RESERVED_DEFAULT); DFSConfigKeys.DFS_DATANODE_DU_RESERVED_DEFAULT);
this.reservedForRbw = new AtomicLong(0L); this.reservedForReplicas = new AtomicLong(0L);
this.currentDir = currentDir; this.currentDir = currentDir;
File parent = currentDir.getParentFile(); File parent = currentDir.getParentFile();
this.usage = new DF(parent, conf); this.usage = new DF(parent, conf);
@ -353,8 +354,9 @@ public class FsVolumeImpl implements FsVolumeSpi {
*/ */
@Override @Override
public long getAvailable() throws IOException { public long getAvailable() throws IOException {
long remaining = getCapacity() - getDfsUsed() - reservedForRbw.get(); long remaining = getCapacity() - getDfsUsed() - reservedForReplicas.get();
long available = usage.getAvailable() - reserved - reservedForRbw.get(); long available = usage.getAvailable() - reserved
- reservedForReplicas.get();
if (remaining > available) { if (remaining > available) {
remaining = available; remaining = available;
} }
@ -362,8 +364,13 @@ public class FsVolumeImpl implements FsVolumeSpi {
} }
@VisibleForTesting @VisibleForTesting
public long getReservedForRbw() { public long getReservedForReplicas() {
return reservedForRbw.get(); return reservedForReplicas.get();
}
@VisibleForTesting
long getRecentReserved() {
return recentReserved;
} }
long getReserved(){ long getReserved(){
@ -412,13 +419,20 @@ public class FsVolumeImpl implements FsVolumeSpi {
*/ */
File createTmpFile(String bpid, Block b) throws IOException { File createTmpFile(String bpid, Block b) throws IOException {
checkReference(); checkReference();
reserveSpaceForReplica(b.getNumBytes());
try {
return getBlockPoolSlice(bpid).createTmpFile(b); return getBlockPoolSlice(bpid).createTmpFile(b);
} catch (IOException exception) {
releaseReservedSpace(b.getNumBytes());
throw exception;
}
} }
@Override @Override
public void reserveSpaceForRbw(long bytesToReserve) { public void reserveSpaceForReplica(long bytesToReserve) {
if (bytesToReserve != 0) { if (bytesToReserve != 0) {
reservedForRbw.addAndGet(bytesToReserve); reservedForReplicas.addAndGet(bytesToReserve);
recentReserved = bytesToReserve;
} }
} }
@ -428,14 +442,15 @@ public class FsVolumeImpl implements FsVolumeSpi {
long oldReservation, newReservation; long oldReservation, newReservation;
do { do {
oldReservation = reservedForRbw.get(); oldReservation = reservedForReplicas.get();
newReservation = oldReservation - bytesToRelease; newReservation = oldReservation - bytesToRelease;
if (newReservation < 0) { if (newReservation < 0) {
// Failsafe, this should never occur in practice, but if it does we don't // Failsafe, this should never occur in practice, but if it does we
// want to start advertising more space than we have available. // don't want to start advertising more space than we have available.
newReservation = 0; newReservation = 0;
} }
} while (!reservedForRbw.compareAndSet(oldReservation, newReservation)); } while (!reservedForReplicas.compareAndSet(oldReservation,
newReservation));
} }
} }
@ -779,7 +794,7 @@ public class FsVolumeImpl implements FsVolumeSpi {
*/ */
File createRbwFile(String bpid, Block b) throws IOException { File createRbwFile(String bpid, Block b) throws IOException {
checkReference(); checkReference();
reserveSpaceForRbw(b.getNumBytes()); reserveSpaceForReplica(b.getNumBytes());
try { try {
return getBlockPoolSlice(bpid).createRbwFile(b); return getBlockPoolSlice(bpid).createRbwFile(b);
} catch (IOException exception) { } catch (IOException exception) {
@ -790,16 +805,15 @@ public class FsVolumeImpl implements FsVolumeSpi {
/** /**
* *
* @param bytesReservedForRbw Space that was reserved during * @param bytesReserved Space that was reserved during
* block creation. Now that the block is being finalized we * block creation. Now that the block is being finalized we
* can free up this space. * can free up this space.
* @return * @return
* @throws IOException * @throws IOException
*/ */
File addFinalizedBlock(String bpid, Block b, File addFinalizedBlock(String bpid, Block b, File f, long bytesReserved)
File f, long bytesReservedForRbw)
throws IOException { throws IOException {
releaseReservedSpace(bytesReservedForRbw); releaseReservedSpace(bytesReserved);
return getBlockPoolSlice(bpid).addBlock(b, f); return getBlockPoolSlice(bpid).addBlock(b, f);
} }

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@ -493,7 +493,7 @@ public class SimulatedFSDataset implements FsDatasetSpi<FsVolumeSpi> {
} }
@Override @Override
public void reserveSpaceForRbw(long bytesToReserve) { public void reserveSpaceForReplica(long bytesToReserve) {
} }
@Override @Override

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@ -607,7 +607,7 @@ public class TestDirectoryScanner {
} }
@Override @Override
public void reserveSpaceForRbw(long bytesToReserve) { public void reserveSpaceForReplica(long bytesToReserve) {
} }
@Override @Override

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@ -75,7 +75,7 @@ public class ExternalVolumeImpl implements FsVolumeSpi {
} }
@Override @Override
public void reserveSpaceForRbw(long bytesToReserve) { public void reserveSpaceForReplica(long bytesToReserve) {
} }
@Override @Override

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@ -28,8 +28,10 @@ import static org.apache.hadoop.hdfs.DFSConfigKeys.*;
import static org.hamcrest.core.Is.is; import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertThat; import static org.junit.Assert.assertThat;
import static org.junit.Assert.assertTrue; import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.fail; import static org.junit.Assert.fail;
import org.apache.hadoop.fs.BlockLocation;
import org.apache.hadoop.fs.FSDataOutputStream; import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.Path; import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hdfs.*; import org.apache.hadoop.hdfs.*;
@ -60,10 +62,10 @@ import javax.management.ObjectName;
/** /**
* Ensure that the DN reserves disk space equivalent to a full block for * Ensure that the DN reserves disk space equivalent to a full block for
* replica being written (RBW). * replica being written (RBW) & Replica being copied from another DN.
*/ */
public class TestRbwSpaceReservation { public class TestSpaceReservation {
static final Log LOG = LogFactory.getLog(TestRbwSpaceReservation.class); static final Log LOG = LogFactory.getLog(TestSpaceReservation.class);
private static final int DU_REFRESH_INTERVAL_MSEC = 500; private static final int DU_REFRESH_INTERVAL_MSEC = 500;
private static final int STORAGES_PER_DATANODE = 1; private static final int STORAGES_PER_DATANODE = 1;
@ -165,14 +167,14 @@ public class TestRbwSpaceReservation {
int bytesWritten = buffer.length; int bytesWritten = buffer.length;
// Check that space was reserved for a full block minus the bytesWritten. // Check that space was reserved for a full block minus the bytesWritten.
assertThat(singletonVolume.getReservedForRbw(), assertThat(singletonVolume.getReservedForReplicas(),
is((long) fileBlockSize - bytesWritten)); is((long) fileBlockSize - bytesWritten));
out.close(); out.close();
out = null; out = null;
// Check that the reserved space has been released since we closed the // Check that the reserved space has been released since we closed the
// file. // file.
assertThat(singletonVolume.getReservedForRbw(), is(0L)); assertThat(singletonVolume.getReservedForReplicas(), is(0L));
// Reopen the file for appends and write 1 more byte. // Reopen the file for appends and write 1 more byte.
out = fs.append(path); out = fs.append(path);
@ -182,7 +184,7 @@ public class TestRbwSpaceReservation {
// Check that space was again reserved for a full block minus the // Check that space was again reserved for a full block minus the
// bytesWritten so far. // bytesWritten so far.
assertThat(singletonVolume.getReservedForRbw(), assertThat(singletonVolume.getReservedForReplicas(),
is((long) fileBlockSize - bytesWritten)); is((long) fileBlockSize - bytesWritten));
// Write once again and again verify the available space. This ensures // Write once again and again verify the available space. This ensures
@ -191,7 +193,7 @@ public class TestRbwSpaceReservation {
out.write(buffer); out.write(buffer);
out.hsync(); out.hsync();
bytesWritten += buffer.length; bytesWritten += buffer.length;
assertThat(singletonVolume.getReservedForRbw(), assertThat(singletonVolume.getReservedForReplicas(),
is((long) fileBlockSize - bytesWritten)); is((long) fileBlockSize - bytesWritten));
} finally { } finally {
if (out != null) { if (out != null) {
@ -282,7 +284,7 @@ public class TestRbwSpaceReservation {
GenericTestUtils.waitFor(new Supplier<Boolean>() { GenericTestUtils.waitFor(new Supplier<Boolean>() {
@Override @Override
public Boolean get() { public Boolean get() {
return (volume.getReservedForRbw() == 0); return (volume.getReservedForReplicas() == 0);
} }
}, 500, Integer.MAX_VALUE); // Wait until the test times out. }, 500, Integer.MAX_VALUE); // Wait until the test times out.
} }
@ -324,12 +326,30 @@ public class TestRbwSpaceReservation {
} }
// Ensure RBW space reserved is released // Ensure RBW space reserved is released
assertTrue("Expected ZERO but got " + fsVolumeImpl.getReservedForRbw(), assertTrue(
fsVolumeImpl.getReservedForRbw() == 0); "Expected ZERO but got " + fsVolumeImpl.getReservedForReplicas(),
fsVolumeImpl.getReservedForReplicas() == 0);
// Reserve some bytes to verify double clearing space should't happen
fsVolumeImpl.reserveSpaceForReplica(1000);
try {
// Write 1 byte to the file
FSDataOutputStream os = fs.create(new Path("/" + methodName + ".02.dat"),
replication);
os.write(new byte[1]);
os.hsync();
os.close();
fail("Expecting IOException file creation failure");
} catch (IOException e) {
// Exception can be ignored (expected)
}
// Ensure RBW space reserved is released only once
assertTrue(fsVolumeImpl.getReservedForReplicas() == 1000);
} }
@Test(timeout = 30000) @Test(timeout = 30000)
public void testRBWInJMXBean() throws Exception { public void testReservedSpaceInJMXBean() throws Exception {
final short replication = 1; final short replication = 1;
startCluster(BLOCK_SIZE, replication, -1); startCluster(BLOCK_SIZE, replication, -1);
@ -348,7 +368,111 @@ public class TestRbwSpaceReservation {
final String volumeInfo = (String) mbs.getAttribute(mxbeanName, final String volumeInfo = (String) mbs.getAttribute(mxbeanName,
"VolumeInfo"); "VolumeInfo");
assertTrue(volumeInfo.contains("reservedSpaceForRBW")); // verify reserved space for Replicas in JMX bean volume info
assertTrue(volumeInfo.contains("reservedSpaceForReplicas"));
}
}
@Test(timeout = 300000)
public void testTmpSpaceReserve() throws Exception {
final short replication = 2;
startCluster(BLOCK_SIZE, replication, -1);
final int byteCount1 = 100;
final int byteCount2 = 200;
final String methodName = GenericTestUtils.getMethodName();
// Test positive scenario
{
final Path file = new Path("/" + methodName + ".01.dat");
try (FSDataOutputStream os = fs.create(file, (short) 1)) {
// Write test data to the file
os.write(new byte[byteCount1]);
os.hsync();
}
BlockLocation[] blockLocations = fs.getFileBlockLocations(file, 0, 10);
String firstReplicaNode = blockLocations[0].getNames()[0];
int newReplicaDNIndex = 0;
if (firstReplicaNode.equals(cluster.getDataNodes().get(0)
.getDisplayName())) {
newReplicaDNIndex = 1;
}
FsVolumeImpl fsVolumeImpl = (FsVolumeImpl) cluster.getDataNodes()
.get(newReplicaDNIndex).getFSDataset().getFsVolumeReferences().get(0);
performReReplication(file, true);
assertEquals("Wrong reserve space for Tmp ", byteCount1,
fsVolumeImpl.getRecentReserved());
assertEquals("Reserved Tmp space is not released", 0,
fsVolumeImpl.getReservedForReplicas());
}
// Test when file creation fails
{
final Path file = new Path("/" + methodName + ".01.dat");
try (FSDataOutputStream os = fs.create(file, (short) 1)) {
// Write test data to the file
os.write(new byte[byteCount2]);
os.hsync();
}
BlockLocation[] blockLocations = fs.getFileBlockLocations(file, 0, 10);
String firstReplicaNode = blockLocations[0].getNames()[0];
int newReplicaDNIndex = 0;
if (firstReplicaNode.equals(cluster.getDataNodes().get(0)
.getDisplayName())) {
newReplicaDNIndex = 1;
}
BlockPoolSlice blockPoolSlice = Mockito.mock(BlockPoolSlice.class);
Mockito.when(blockPoolSlice.createTmpFile((Block) Mockito.any()))
.thenThrow(new IOException("Synthetic IO Exception Throgh MOCK"));
final FsVolumeImpl fsVolumeImpl = (FsVolumeImpl) cluster.getDataNodes()
.get(newReplicaDNIndex).getFSDataset().getFsVolumeReferences().get(0);
// Reserve some bytes to verify double clearing space should't happen
fsVolumeImpl.reserveSpaceForReplica(1000);
Field field = FsVolumeImpl.class.getDeclaredField("bpSlices");
field.setAccessible(true);
@SuppressWarnings("unchecked")
Map<String, BlockPoolSlice> bpSlices = (Map<String, BlockPoolSlice>) field
.get(fsVolumeImpl);
bpSlices.put(fsVolumeImpl.getBlockPoolList()[0], blockPoolSlice);
performReReplication(file, false);
assertEquals("Wrong reserve space for Tmp ", byteCount2,
fsVolumeImpl.getRecentReserved());
assertEquals("Tmp space is not released OR released twice", 1000,
fsVolumeImpl.getReservedForReplicas());
}
}
private void performReReplication(Path filePath, boolean waitForSuccess)
throws Exception {
fs.setReplication(filePath, (short) 2);
Thread.sleep(4000);
BlockLocation[] blockLocations = fs.getFileBlockLocations(filePath, 0, 10);
if (waitForSuccess) {
// Wait for the re replication
while (blockLocations[0].getNames().length < 2) {
Thread.sleep(2000);
blockLocations = fs.getFileBlockLocations(filePath, 0, 10);
}
} }
} }
@ -387,7 +511,7 @@ public class TestRbwSpaceReservation {
" files and hit " + numFailures + " failures"); " files and hit " + numFailures + " failures");
// Check no space was leaked. // Check no space was leaked.
assertThat(singletonVolume.getReservedForRbw(), is(0L)); assertThat(singletonVolume.getReservedForReplicas(), is(0L));
} }
private static class Writer extends Daemon { private static class Writer extends Daemon {