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<section id="checksums">
<h1><span class="section-number">2.5. </span>Checksums<a class="headerlink" href="#checksums" title="Link to this heading">ΒΆ</a></h1>
<p>Starting in early 2012, metadata checksums were added to all major ext4
and jbd2 data structures. The associated feature flag is metadata_csum.
The desired checksum algorithm is indicated in the superblock, though as
of October 2012 the only supported algorithm is crc32c. Some data
structures did not have space to fit a full 32-bit checksum, so only the
lower 16 bits are stored. Enabling the 64bit feature increases the data
structure size so that full 32-bit checksums can be stored for many data
structures. However, existing 32-bit filesystems cannot be extended to
enable 64bit mode, at least not without the experimental resize2fs
patches to do so.</p>
<p>Existing filesystems can have checksumming added by running
<code class="docutils literal notranslate"><span class="pre">tune2fs</span> <span class="pre">-O</span> <span class="pre">metadata_csum</span></code> against the underlying device. If tune2fs
encounters directory blocks that lack sufficient empty space to add a
checksum, it will request that you run <code class="docutils literal notranslate"><span class="pre">e2fsck</span> <span class="pre">-D</span></code> to have the
directories rebuilt with checksums. This has the added benefit of
removing slack space from the directory files and rebalancing the htree
indexes. If you _ignore_ this step, your directories will not be
protected by a checksum!</p>
<p>The following table describes the data elements that go into each type
of checksum. The checksum function is whatever the superblock describes
(crc32c as of October 2013) unless noted otherwise.</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 25.6%" />
<col style="width: 10.3%" />
<col style="width: 64.1%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Metadata</p></th>
<th class="head"><p>Length</p></th>
<th class="head"><p>Ingredients</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>Superblock</p></td>
<td><p>__le32</p></td>
<td><p>The entire superblock up to the checksum field. The UUID lives inside
the superblock.</p></td>
</tr>
<tr class="row-odd"><td><p>MMP</p></td>
<td><p>__le32</p></td>
<td><p>UUID + the entire MMP block up to the checksum field.</p></td>
</tr>
<tr class="row-even"><td><p>Extended Attributes</p></td>
<td><p>__le32</p></td>
<td><p>UUID + the entire extended attribute block. The checksum field is set to
zero.</p></td>
</tr>
<tr class="row-odd"><td><p>Directory Entries</p></td>
<td><p>__le32</p></td>
<td><p>UUID + inode number + inode generation + the directory block up to the
fake entry enclosing the checksum field.</p></td>
</tr>
<tr class="row-even"><td><p>HTREE Nodes</p></td>
<td><p>__le32</p></td>
<td><p>UUID + inode number + inode generation + all valid extents + HTREE tail.
The checksum field is set to zero.</p></td>
</tr>
<tr class="row-odd"><td><p>Extents</p></td>
<td><p>__le32</p></td>
<td><p>UUID + inode number + inode generation + the entire extent block up to
the checksum field.</p></td>
</tr>
<tr class="row-even"><td><p>Bitmaps</p></td>
<td><p>__le32 or __le16</p></td>
<td><p>UUID + the entire bitmap. Checksums are stored in the group descriptor,
and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)</p></td>
</tr>
<tr class="row-odd"><td><p>Inodes</p></td>
<td><p>__le32</p></td>
<td><p>UUID + inode number + inode generation + the entire inode. The checksum
field is set to zero. Each inode has its own checksum.</p></td>
</tr>
<tr class="row-even"><td><p>Group Descriptors</p></td>
<td><p>__le16</p></td>
<td><p>If metadata_csum, then UUID + group number + the entire descriptor;
else if gdt_csum, then crc16(UUID + group number + the entire
descriptor). In all cases, only the lower 16 bits are stored.</p></td>
</tr>
</tbody>
</table>
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