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  <properties>
    <title>HFDS Snapshots</title>
  </properties>

  <body>

  <h1>HDFS Snapshots</h1>
  <macro name="toc">
    <param name="section" value="0"/>
    <param name="fromDepth" value="0"/>
    <param name="toDepth" value="4"/>
  </macro>

  <section name="Overview" id="Overview">
  <p>
    HDFS Snapshots are read-only point-in-time copies of the file system.
    Snapshots can be taken on a subtree of the file system or the entire file system.
    Some common use cases of snapshots are data backup, protection against user errors
    and disaster recovery.
  </p>

  <p>
    The implementation of HDFS Snapshots is efficient:
  </p>
  <ul>
    <li>Snapshot creation is instantaneous:
        the cost is <em>O(1)</em> excluding the inode lookup time.</li>
    <li>Additional memory is used only when modifications are made relative to a snapshot:
        memory usage is <em>O(M)</em>,
        where <em>M</em> is the number of modified files/directories.</li>
    <li>Blocks in datanodes are not copied:
        the snapshot files record the block list and the file size.
        There is no data copying.</li>
    <li>Snapshots do not adversely affect regular HDFS operations:
        modifications are recorded in reverse chronological order
        so that the current data can be accessed directly.
        The snapshot data is computed by subtracting the modifications
        from the current data.</li>
  </ul>

  <subsection name="Snapshottable Directories" id="SnapshottableDirectories">
  <p>
    Snapshots can be taken on any directory once the directory has been set as
    <em>snapshottable</em>.
    A snapshottable directory is able to accommodate 65,536 simultaneous snapshots.
    There is no limit on the number of snapshottable directories.
    Administrators may set any directory to be snapshottable.
    If there are snapshots in a snapshottable directory,
    the directory can be neither deleted nor renamed
    before all the snapshots are deleted.
  </p>
<!--
  <p>
    Nested snapshottable directories are currently not allowed.
    In other words, a directory cannot be set to snapshottable
    if one of its ancestors is a snapshottable directory.
  </p>
-->
  </subsection>

  <subsection name="Snapshot Paths" id="SnapshotPaths">
  <p>
    For a snapshottable directory,
    the path component <em>".snapshot"</em> is used for accessing its snapshots.
    Suppose <code>/foo</code> is a snapshottable directory,
    <code>/foo/bar</code> is a file/directory in <code>/foo</code>,
    and <code>/foo</code> has a snapshot <code>s0</code>.
    Then, the path <source>/foo/.snapshot/s0/bar</source>
    refers to the snapshot copy of <code>/foo/bar</code>.
    The usual API and CLI can work with the ".snapshot" paths.
    The following are some examples.
  </p>
  <ul>
    <li>Listing all the snapshots under a snapshottable directory:
      <source>hdfs dfs -ls /foo/.snapshot</source></li>
    <li>Listing the files in snapshot <code>s0</code>:
      <source>hdfs dfs -ls /foo/.snapshot/s0</source></li>
    <li>Copying a file from snapshot <code>s0</code>:
      <source>hdfs dfs -cp /foo/.snapshot/s0/bar /tmp</source></li>
  </ul>
  <p>
    The name ".snapshot" is now a reserved file name in HDFS
    so that users cannot create a file/directory with ".snapshot" as the name.
    If ".snapshot" is used in a previous version of HDFS, it must be renamed before upgrade;
    otherwise, upgrade will fail. 
  </p>
  </subsection>
  </section>

  <section name="Snapshot Operations" id="SnapshotOperations">
  <subsection name="Administrator Operations" id="AdministratorOperations">
  <p>
    The operations described in this section require superuser privilege.
  </p>

  <h4>Allow Snapshots</h4>
  <p>
    Allowing snapshots of a directory to be created.
    If the operation completes successfully, the directory becomes snapshottable.
  </p>
  <ul>
    <li>Command:
      <source>hdfs dfsadmin -allowSnapshot &lt;path&gt;</source></li>
    <li>Arguments:<table>
      <tr><td>path</td><td>The path of the snapshottable directory.</td></tr>
    </table></li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>void allowSnapshot(Path path)</code> in <code>HdfsAdmin</code>.
  </p>

  <h4>Disallow Snapshots</h4>
  <p>
    Disallowing snapshots of a directory to be created.
    All snapshots of the directory must be deleted before disallowing snapshots.
  </p>
  <ul>
    <li>Command:
      <source>hdfs dfsadmin -disallowSnapshot &lt;path&gt;</source></li>
    <li>Arguments:<table>
      <tr><td>path</td><td>The path of the snapshottable directory.</td></tr>
    </table></li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>void disallowSnapshot(Path path)</code> in <code>HdfsAdmin</code>.
  </p>
  </subsection>

  <subsection name="User Operations" id="UserOperations">
  <p>
    The section describes user operations.
    Note that HDFS superuser can perform all the operations
    without satisfying the permission requirement in the individual operations.
  </p>

  <h4>Create Snapshots</h4>
  <p>
    Create a snapshot of a snapshottable directory.
    This operation requires owner privilege of the snapshottable directory.
  </p>
  <ul>
    <li>Command:
      <source>hdfs dfs -createSnapshot &lt;path&gt; [&lt;snapshotName&gt;]</source></li>
    <li>Arguments:<table>
      <tr><td>path</td><td>The path of the snapshottable directory.</td></tr>
      <tr><td>snapshotName</td><td>
        The snapshot name, which is an optional argument.
        When it is omitted, a default name is generated using a timestamp with the format
        <code>"'s'yyyyMMdd-HHmmss.SSS"</code>, e.g. "s20130412-151029.033".
      </td></tr>
    </table></li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>Path createSnapshot(Path path)</code> and
    <code>Path createSnapshot(Path path, String snapshotName)</code>
    in <a href="../../api/org/apache/hadoop/fs/FileSystem.html"><code>FileSystem</code></a>.
    The snapshot path is returned in these methods.
  </p>

  <h4>Delete Snapshots</h4>
  <p>
    Delete a snapshot of from a snapshottable directory.
    This operation requires owner privilege of the snapshottable directory.
  </p>
  <ul>
    <li>Command:
      <source>hdfs dfs -deleteSnapshot &lt;path&gt; &lt;snapshotName&gt;</source></li>
    <li>Arguments:<table>
      <tr><td>path</td><td>The path of the snapshottable directory.</td></tr>
      <tr><td>snapshotName</td><td>The snapshot name.</td></tr>
    </table></li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>void deleteSnapshot(Path path, String snapshotName)</code>
    in <a href="../../api/org/apache/hadoop/fs/FileSystem.html"><code>FileSystem</code></a>.
  </p>

  <h4>Rename Snapshots</h4>
  <p>
    Rename a snapshot.
    This operation requires owner privilege of the snapshottable directory.
  </p>
  <ul>
    <li>Command:
      <source>hdfs dfs -renameSnapshot &lt;path&gt; &lt;oldName&gt; &lt;newName&gt;</source></li>
    <li>Arguments:<table>
      <tr><td>path</td><td>The path of the snapshottable directory.</td></tr>
      <tr><td>oldName</td><td>The old snapshot name.</td></tr>
      <tr><td>newName</td><td>The new snapshot name.</td></tr>
    </table></li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>void renameSnapshot(Path path, String oldName, String newName)</code>
    in <a href="../../api/org/apache/hadoop/fs/FileSystem.html"><code>FileSystem</code></a>.
  </p>

  <h4>Get Snapshottable Directory Listing</h4>
  <p>
    Get all the snapshottable directories where the current user has permission to take snapshtos.
  </p>
  <ul>
    <li>Command:
      <source>hdfs lsSnapshottableDir</source></li>
    <li>Arguments: none</li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>SnapshottableDirectoryStatus[] getSnapshottableDirectoryListing()</code>
    in <code>DistributedFileSystem</code>.
  </p>

  <h4>Get Snapshots Difference Report</h4>
  <p>
    Get the differences between two snapshots.
    This operation requires read access privilege for all files/directories in both snapshots.
  </p>
  <ul>
    <li>Command:
      <source>hdfs snapshotDiff &lt;path&gt; &lt;fromSnapshot&gt; &lt;toSnapshot&gt;</source></li>
    <li>Arguments:<table>
      <tr><td>path</td><td>The path of the snapshottable directory.</td></tr>
      <tr><td>fromSnapshot</td><td>The name of the starting snapshot.</td></tr>
      <tr><td>toSnapshot</td><td>The name of the ending snapshot.</td></tr>
    </table></li>
  </ul>
  <p>
    See also the corresponding Java API
    <code>SnapshotDiffReport getSnapshotDiffReport(Path path, String fromSnapshot, String toSnapshot)</code>
    in <code>DistributedFileSystem</code>.
  </p>

  </subsection>
  </section>

  </body>
</document>