hadoop/hadoop-hdfs-project/hadoop-hdfs/src/site/apt/HdfsImageViewer.apt.vm

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  ---
  Offline Image Viewer Guide
  ---
  ---
  ${maven.build.timestamp}

Offline Image Viewer Guide

%{toc|section=1|fromDepth=0}

* Overview

   The Offline Image Viewer is a tool to dump the contents of hdfs fsimage
   files to a human-readable format and provide read-only WebHDFS API
   in order to allow offline analysis and examination of an Hadoop cluster's
   namespace. The tool is able to process very large image files relatively
   quickly. The tool handles the layout formats that were included with Hadoop
   versions 2.4 and up. If you want to handle older layout formats, you can
   use the Offline Image Viewer of Hadoop 2.3 or {{oiv_legacy Command}}.
   If the tool is not able to process an image file, it will exit cleanly.
   The Offline Image Viewer does not require a Hadoop cluster to be running;
   it is entirely offline in its operation.

   The Offline Image Viewer provides several output processors:

   [[1]] Web is the default output processor. It launches a HTTP server
      that exposes read-only WebHDFS API. Users can investigate the namespace
      interactively by using HTTP REST API.

   [[2]] XML creates an XML document of the fsimage and includes all of the
      information within the fsimage, similar to the lsr processor. The
      output of this processor is amenable to automated processing and
      analysis with XML tools. Due to the verbosity of the XML syntax,
      this processor will also generate the largest amount of output.

   [[3]] FileDistribution is the tool for analyzing file sizes in the
      namespace image. In order to run the tool one should define a range
      of integers [0, maxSize] by specifying maxSize and a step. The
      range of integers is divided into segments of size step: [0, s[1],
      ..., s[n-1], maxSize], and the processor calculates how many files
      in the system fall into each segment [s[i-1], s[i]). Note that
      files larger than maxSize always fall into the very last segment.
      The output file is formatted as a tab separated two column table:
      Size and NumFiles. Where Size represents the start of the segment,
      and numFiles is the number of files form the image which size falls
      in this segment.

* Usage

** Web Processor

   Web processor launches a HTTP server which exposes read-only WebHDFS API.
   Users can specify the address to listen by -addr option (default by
   localhost:5978).

----
   bash$ bin/hdfs oiv -i fsimage
   14/04/07 13:25:14 INFO offlineImageViewer.WebImageViewer: WebImageViewer
   started. Listening on /127.0.0.1:5978. Press Ctrl+C to stop the viewer.
----

   Users can access the viewer and get the information of the fsimage by
   the following shell command:

----
   bash$ bin/hdfs dfs -ls webhdfs://127.0.0.1:5978/
   Found 2 items
   drwxrwx---   - root supergroup          0 2014-03-26 20:16 webhdfs://127.0.0.1:5978/tmp
   drwxr-xr-x   - root supergroup          0 2014-03-31 14:08 webhdfs://127.0.0.1:5978/user
----

   To get the information of all the files and directories, you can simply use
   the following command:

----
   bash$ bin/hdfs dfs -ls -R webhdfs://127.0.0.1:5978/
----

   Users can also get JSON formatted FileStatuses via HTTP REST API.

----
   bash$ curl -i http://127.0.0.1:5978/webhdfs/v1/?op=liststatus
   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Length: 252

   {"FileStatuses":{"FileStatus":[
   {"fileId":16386,"accessTime":0,"replication":0,"owner":"theuser","length":0,"permission":"755","blockSize":0,"modificationTime":1392772497282,"type":"DIRECTORY","group":"supergroup","childrenNum":1,"pathSuffix":"user"}
   ]}}
----

   The Web processor now supports the following operations:

   * {{{./WebHDFS.html#List_a_Directory}LISTSTATUS}}

   * {{{./WebHDFS.html#Status_of_a_FileDirectory}GETFILESTATUS}}

   * {{{./WebHDFS.html#Get_ACL_Status}GETACLSTATUS}}

** XML Processor

   XML Processor is used to dump all the contents in the fsimage. Users can
   specify input and output file via -i and -o command-line.

----
   bash$ bin/hdfs oiv -p XML -i fsimage -o fsimage.xml
----

   This will create a file named fsimage.xml contains all the information in
   the fsimage. For very large image files, this process may take several
   minutes.

   Applying the Offline Image Viewer with XML processor would result in the
   following output:

----
   <?xml version="1.0"?>
   <fsimage>
   <NameSection>
     <genstampV1>1000</genstampV1>
     <genstampV2>1002</genstampV2>
     <genstampV1Limit>0</genstampV1Limit>
     <lastAllocatedBlockId>1073741826</lastAllocatedBlockId>
     <txid>37</txid>
   </NameSection>
   <INodeSection>
     <lastInodeId>16400</lastInodeId>
     <inode>
       <id>16385</id>
       <type>DIRECTORY</type>
       <name></name>
       <mtime>1392772497282</mtime>
       <permission>theuser:supergroup:rwxr-xr-x</permission>
       <nsquota>9223372036854775807</nsquota>
       <dsquota>-1</dsquota>
     </inode>
   ...remaining output omitted...
----

* Options

*-----------------------:-----------------------------------+
| <<Flag>>              | <<Description>>                   |
*-----------------------:-----------------------------------+
| <<<-i>>>\|<<<--inputFile>>> <input file> | Specify the input fsimage file
|                       | to process. Required.
*-----------------------:-----------------------------------+
| <<<-o>>>\|<<<--outputFile>>> <output file> | Specify the output filename,
|                       | if the specified output processor generates one. If
|                       | the specified file already exists, it is silently
|                       | overwritten. (output to stdout by default)
*-----------------------:-----------------------------------+
| <<<-p>>>\|<<<--processor>>> <processor> | Specify the image processor to
|                       | apply against the image file. Currently valid options
|                       | are Web (default), XML and FileDistribution.
*-----------------------:-----------------------------------+
| <<<-addr>>> <address> | Specify the address(host:port) to listen.
|                       | (localhost:5978 by default). This option is used with
|                       | Web processor.
*-----------------------:-----------------------------------+
| <<<-maxSize>>> <size> | Specify the range [0, maxSize] of file sizes to be
|                       | analyzed in bytes (128GB by default). This option is
|                       | used with FileDistribution processor.
*-----------------------:-----------------------------------+
| <<<-step>>> <size>    | Specify the granularity of the distribution in bytes
|                       | (2MB by default). This option is used with
|                       | FileDistribution processor.
*-----------------------:-----------------------------------+
| <<<-h>>>\|<<<--help>>>| Display the tool usage and help information and
|                       | exit.
*-----------------------:-----------------------------------+

* Analyzing Results

   The Offline Image Viewer makes it easy to gather large amounts of data
   about the hdfs namespace. This information can then be used to explore
   file system usage patterns or find specific files that match arbitrary
   criteria, along with other types of namespace analysis.

* oiv_legacy Command

   Due to the internal layout changes introduced by the ProtocolBuffer-based
   fsimage ({{{https://issues.apache.org/jira/browse/HDFS-5698}HDFS-5698}}),
   OfflineImageViewer consumes excessive amount of memory and loses some
   functions such as Indented and Delimited processor. If you want to process
   without large amount of memory or use these processors, you can use
   <<<oiv_legacy>>> command (same as <<<oiv>>> in Hadoop 2.3).

** Usage

   1. Set <<<dfs.namenode.legacy-oiv-image.dir>>> to an appropriate directory
      to make standby NameNode or SecondaryNameNode save its namespace in the
      old fsimage format during checkpointing.

   2. Use <<<oiv_legacy>>> command to the old format fsimage.

----
   bash$ bin/hdfs oiv_legacy -i fsimage_old -o output
----

** Options

*-----------------------:-----------------------------------+
| <<Flag>>              | <<Description>>                   |
*-----------------------:-----------------------------------+
| <<<-i>>>\|<<<--inputFile>>> <input file> | Specify the input fsimage file to
|                       | process. Required.
*-----------------------:-----------------------------------+
| <<<-o>>>\|<<<--outputFile>>> <output file> | Specify the output filename, if
|                       | the specified output processor generates one. If the
|                       | specified file already exists, it is silently
|                       | overwritten. Required.
*-----------------------:-----------------------------------+
| <<<-p>>>\|<<<--processor>>> <processor> | Specify the image processor to
|                       | apply against the image file. Valid options are
|                       | Ls (default), XML, Delimited, Indented, and
|                       | FileDistribution.
*-----------------------:-----------------------------------+
| <<<-skipBlocks>>>     | Do not enumerate individual blocks within files. This
|                       | may save processing time and outfile file space on
|                       | namespaces with very large files. The Ls processor
|                       | reads the blocks to correctly determine file sizes
|                       | and ignores this option.
*-----------------------:-----------------------------------+
| <<<-printToScreen>>>  | Pipe output of processor to console as well as
|                       | specified file. On extremely large namespaces, this
|                       | may increase processing time by an order of
|                       | magnitude.
*-----------------------:-----------------------------------+
| <<<-delimiter>>> <arg>| When used in conjunction with the Delimited
|                       | processor, replaces the default tab delimiter with
|                       | the string specified by <arg>.
*-----------------------:-----------------------------------+
| <<<-h>>>\|<<<--help>>>| Display the tool usage and help information and exit.
*-----------------------:-----------------------------------+