HADOOP-18596. Distcp -update to use modification time while checking for file skip.

hadoop-tools/hadoop-distcp/src/main/java/org/apache/hadoop/tools/DistCpConstants.java

@@ -142,6 +142,26 @@ private DistCpConstants() {
       "distcp.blocks.per.chunk";
 
   public static final String CONF_LABEL_USE_ITERATOR = "distcp.use.iterator";
+
+  /**
+   * Enabling {@code distcp -update} to use modification time of source and
+   * target file to check while copying same file with same size but
+   * different content.
+   *
+   * The check would verify if the target file is perceived as older than the
+   * source then it indicates that the source has been recently updated and it
+   * is a newer version than what was synced, so we should not skip the copy.
+   * {@value}
+   */
+  public static final String CONF_LABEL_UPDATE_MOD_TIME =
+      "distcp.update.modification.time";
+
+  /**
+   * Default value for 'distcp.update.modification.time' configuration.
+   */
+  public static final boolean CONF_LABEL_UPDATE_MOD_TIME_DEFAULT =
+      true;
+
   /**
    * Constants for DistCp return code to shell / consumer of ToolRunner's run
    */

hadoop-tools/hadoop-distcp/src/main/java/org/apache/hadoop/tools/mapred/CopyMapper.java

@@ -41,6 +41,8 @@
 import org.apache.hadoop.tools.util.DistCpUtils;
 import org.apache.hadoop.util.StringUtils;
 
+import static org.apache.hadoop.tools.DistCpConstants.CONF_LABEL_UPDATE_MOD_TIME_DEFAULT;
+
 /**
  * Mapper class that executes the DistCp copy operation.
  * Implements the o.a.h.mapreduce.Mapper interface.
@@ -74,6 +76,15 @@ static enum FileAction {
     OVERWRITE,    // Overwrite the whole file
   }
 
+  /**
+   * Indicates the checksum comparison result.
+   */
+  public enum ChecksumComparison {
+    TRUE,           // checksum comparison is compatible and true.
+    FALSE,          // checksum comparison is compatible and false.
+    INCOMPATIBLE,   // checksum comparison is not compatible.
+  }
+
   private static Logger LOG = LoggerFactory.getLogger(CopyMapper.class);
 
   private Configuration conf;
@@ -85,6 +96,7 @@ static enum FileAction {
   private boolean append = false;
   private boolean verboseLog = false;
   private boolean directWrite = false;
+  private boolean useModTimeToUpdate;
   private EnumSet<FileAttribute> preserve = EnumSet.noneOf(FileAttribute.class);
 
   private FileSystem targetFS = null;
@@ -114,6 +126,9 @@ public void setup(Context context) throws IOException, InterruptedException {
         PRESERVE_STATUS.getConfigLabel()));
     directWrite = conf.getBoolean(
         DistCpOptionSwitch.DIRECT_WRITE.getConfigLabel(), false);
+    useModTimeToUpdate =
+        conf.getBoolean(DistCpConstants.CONF_LABEL_UPDATE_MOD_TIME,
+            CONF_LABEL_UPDATE_MOD_TIME_DEFAULT);
 
     targetWorkPath = new Path(conf.get(DistCpConstants.CONF_LABEL_TARGET_WORK_PATH));
     Path targetFinalPath = new Path(conf.get(
@@ -354,13 +369,65 @@ private boolean canSkip(FileSystem sourceFS, CopyListingFileStatus source,
     boolean sameLength = target.getLen() == source.getLen();
     boolean sameBlockSize = source.getBlockSize() == target.getBlockSize()
         || !preserve.contains(FileAttribute.BLOCKSIZE);
+    // Skip the copy if a 0 size file is being copied.
+    if (sameLength && source.getLen() == 0) {
+      return true;
+    }
+    // If the src and target file have same size and block size, we would
+    // check if the checkCrc flag is enabled or not. If enabled, and the
+    // modTime comparison is enabled then return true if target file is older
+    // than the source file, since this indicates that the target file is
+    // recently updated and the source is not changed more recently than the
+    // update, we can skip the copy else we would copy.
+    // If skipCrc flag is disabled, we would check the checksum comparison
+    // which is an enum representing 3 values, of which if the comparison
+    // returns NOT_COMPATIBLE, we'll try to check modtime again, else return
+    // the result of checksum comparison which are compatible(true or false).
+    //
+    // Note: Different object stores can have different checksum algorithms
+    // resulting in no checksum comparison that results in return true
+    // always, having the modification time enabled can help in these
+    // scenarios to not incorrectly skip a copy. Refer: HADOOP-18596.
+
     if (sameLength && sameBlockSize) {
-      return skipCrc ||
-          DistCpUtils.checksumsAreEqual(sourceFS, source.getPath(), null,
-              targetFS, target.getPath(), source.getLen());
-    } else {
-      return false;
+      if (skipCrc) {
+        return maybeUseModTimeToCompare(source, target);
+      } else {
+        ChecksumComparison checksumComparison = DistCpUtils
+            .checksumsAreEqual(sourceFS, source.getPath(), null,
+                targetFS, target.getPath(), source.getLen());
+        LOG.debug("Result of checksum comparison between src {} and target "
+            + "{} : {}", source, target, checksumComparison);
+        if (checksumComparison.equals(ChecksumComparison.INCOMPATIBLE)) {
+          return maybeUseModTimeToCompare(source, target);
+        }
+        // if skipCrc is disabled and checksumComparison is compatible we
+        // need not check the mod time.
+        return checksumComparison.equals(ChecksumComparison.TRUE);
+      }
+    }
+    return false;
+  }
+
+  /**
+   * If the mod time comparison is enabled, check the mod time else return
+   * false.
+   * Comparison: If the target file perceives to have greater or equal mod time
+   * (older) than the source file, we can assume that there has been no new
+   * changes that occurred in the source file, hence we should return true to
+   * skip the copy of the file.
+   *
+   * @param source Source fileStatus.
+   * @param target Target fileStatus.
+   * @return boolean representing result of modTime check.
+   */
+  private boolean maybeUseModTimeToCompare(
+      CopyListingFileStatus source, FileStatus target) {
+    if (useModTimeToUpdate) {
+      return source.getModificationTime() <= target.getModificationTime();
     }
+    // if we cannot check mod time, return true (skip the copy).
+    return true;
   }
 
   @Override

hadoop-tools/hadoop-distcp/src/main/java/org/apache/hadoop/tools/util/DistCpUtils.java

@@ -41,6 +41,7 @@
 import org.apache.hadoop.tools.CopyListingFileStatus;
 import org.apache.hadoop.tools.DistCpContext;
 import org.apache.hadoop.tools.DistCpOptions.FileAttribute;
+import org.apache.hadoop.tools.mapred.CopyMapper;
 import org.apache.hadoop.tools.mapred.UniformSizeInputFormat;
 import org.apache.hadoop.util.StringUtils;
 
@@ -568,10 +569,12 @@ public static String getStringDescriptionFor(long nBytes) {
    * and false otherwise.
    * @throws IOException if there's an exception while retrieving checksums.
    */
-  public static boolean checksumsAreEqual(FileSystem sourceFS, Path source,
-                                          FileChecksum sourceChecksum,
-                                          FileSystem targetFS,
-                                          Path target, long sourceLen)
+  public static CopyMapper.ChecksumComparison checksumsAreEqual(
+      FileSystem sourceFS,
+      Path source,
+      FileChecksum sourceChecksum,
+      FileSystem targetFS,
+      Path target, long sourceLen)
       throws IOException {
     FileChecksum targetChecksum = null;
     try {
@@ -585,8 +588,15 @@ public static boolean checksumsAreEqual(FileSystem sourceFS, Path source,
     } catch (IOException e) {
       LOG.error("Unable to retrieve checksum for " + source + " or " + target, e);
     }
-    return (sourceChecksum == null || targetChecksum == null ||
-            sourceChecksum.equals(targetChecksum));
+    // If the source or target checksum is null, that means there is no
+    // comparison that took place and return not compatible.
+    // else if matched, return compatible with the matched result.
+    if (sourceChecksum == null || targetChecksum == null) {
+      return CopyMapper.ChecksumComparison.INCOMPATIBLE;
+    } else if (sourceChecksum.equals(targetChecksum)) {
+      return CopyMapper.ChecksumComparison.TRUE;
+    }
+    return CopyMapper.ChecksumComparison.FALSE;
   }
 
   /**
@@ -613,8 +623,12 @@ public static void compareFileLengthsAndChecksums(long srcLen,
 
     //At this point, src & dest lengths are same. if length==0, we skip checksum
     if ((srcLen != 0) && (!skipCrc)) {
-      if (!checksumsAreEqual(sourceFS, source, sourceChecksum,
-          targetFS, target, srcLen)) {
+      CopyMapper.ChecksumComparison
+          checksumComparison = checksumsAreEqual(sourceFS, source, sourceChecksum,
+              targetFS, target, srcLen);
+      // If Checksum comparison is false set it to false, else set to true.
+      boolean checksumResult = !checksumComparison.equals(CopyMapper.ChecksumComparison.FALSE);
+      if (!checksumResult) {
         StringBuilder errorMessage =
             new StringBuilder(DistCpConstants.CHECKSUM_MISMATCH_ERROR_MSG)
                 .append(source).append(" and ").append(target).append(".");

hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm

@@ -631,14 +631,37 @@ hadoop distcp -update -numListstatusThreads 20  \
 Because object stores are slow to list files, consider setting the `-numListstatusThreads` option when performing a `-update` operation
 on a large directory tree (the limit is 40 threads).
 
-When `DistCp -update` is used with object stores,
-generally only the modification time and length of the individual files are compared,
-not any checksums. The fact that most object stores do have valid timestamps
-for directories is irrelevant; only the file timestamps are compared.
-However, it is important to have the clock of the client computers close
-to that of the infrastructure, so that timestamps are consistent between
-the client/HDFS cluster and that of the object store. Otherwise, changed files may be
-missed/copied too often.
+When `DistCp -update` is used with object stores, generally only the
+modification time and length of the individual files are compared, not any
+checksums if the checksum algorithm between the two stores is different.
+
+* The `distcp -update` between two object stores with different checksum
+  algorithm compares the modification times of source and target files along
+  with the file size to determine whether to skip the file copy. The behavior
+  is controlled by the property `distcp.update.modification.time`, which is
+  set to true by default. If the source file is more recently modified than
+  the target file, it is assumed that the content has changed, and the file
+  should be updated.
+  We need to ensure that there is no clock skew between the machines.
+  The fact that most object stores do have valid timestamps for directories
+  is irrelevant; only the file timestamps are compared. However, it is
+  important to have the clock of the client computers close to that of the
+  infrastructure, so that timestamps are consistent between the client/HDFS
+  cluster and that of the object store. Otherwise, changed files may be
+  missed/copied too often.
+
+* `distcp.update.modification.time` would only be used if either of the two
+  stores don't have checksum validation resulting in incompatible checksum
+  comparison between the two. Even if the property is set to true, it won't
+  be used if their is valid checksum comparison between the two stores.
+
+To turn off the modification time check, set this in your core-site.xml
+```xml
+<property>
+        <name>distcp.update.modification.time</name>
+        <value>false</value>
+</property>
+```
 
 **Notes**