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<title>Understanding RAID level-6</title>

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<h1>Understanding RAID level-6</h1>

<p>RAID level-6 is similar to <a href="craid5.htm">RAID level-5</a> but with two sets of parity information instead of
one. RAID level-6 <a href="cstripe.htm">stripes</a> blocks of data and parity across all drives in the pool like RAID
level-5, but adds a second set of parity information for each block of data.</p>

<p>
When you assign RAID level-6 to an pool, the capacity of the pool is reduced
for data-parity storage (the exact amount depends on the size of the drives in the pool). The second set of parity information is
added to improve fault tolerance. RAID level-6 can handle two simultaneous drive
failures, where other single RAID levels can handle, at most, only one.</p>
<p>
RAID level-6 requires a minimum of four drives and supports a maximum of 16 drives. The maximum stripe size
depends on the number of drives in the pool.</p>


<p> The following illustration is an example of a RAID level-6 logical device.
<h2>RAID level-6 example</h2>
<table width="557" border="0" cellpadding="3">
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="344">Start with six physical drives.
    </td>
    <td width="195"><div align="center"><img src="images/raid6a.gif" width="260" height="70">
      </div></td>
  </tr>

  <tr valign=top>
    <td align="LEFT" valign="TOP" width="344">Create a logical device using  four physical
      drives, leaving two for hot spare drives.
      <p>&nbsp;</td>
    <td width="195"><img src="images/raid6c.gif" width="290" height="97"> </td>
  </tr>
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="344">The data is striped across the drives,
      creating blocks in the logical device.
   The storage of the data parity (denoted by <b><sub><font size="5">*</font></sub></b> and <b><sub><font size="5">**</font></sub></b>)
        is striped, and it shifts from drive to drive as it does in RAID level-5.
      </td>
    <td width="195"><img src="images/raid6d.gif" width="290" height="94"> </td>
  </tr>
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="344"> If a physical drive fails in the
      pool, the logical device is degraded but remains fault tolerant.</td>
    <td width="195"><img src="images/raid6e.gif" width="290" height="94"> </td>
  </tr>

  <tr valign=top>
      <td align="LEFT" valign="TOP" width="344"> If a second physical drive fails in the
        pool, the data from the failed drives are reconstructed onto the hot-spare drives, and
        the data for the logical device return to the original striping
        scheme.</td>
      <td width="195">
      <img src="images/raid6f.gif" width="285" height="105">
<!-- alt image
      <img src="images/raid6f-b.gif" width="285" height="115">
-->
      </td>
  </tr>

</table>
<p> RAID level-6 offers the following advantages and disadvantages. <br>
<table  BORDER>
  <tr bgcolor="#C0C0C0">
    <th align="LEFT" valign="BOTTOM" width="50%">Advantages </th>
    <th align="LEFT" valign="BOTTOM" width="50%">Disadvantages </th>
  </tr>
  <tr>
    <td align="LEFT" valign="TOP" width="50%"> <ul>
        <li>100% data protection </li>
        <li>Extremely high data fault tolerance
        </li>
        <li>Can sustain two simultaneous drive failures</li>

      <li>Good solution for mission critical applications</li>
      </ul>
      <br> </td>
    <td align="LEFT" valign="TOP" width="50%"> <ul>
        <li>Lower performance than RAID level-5 because of two parity drives</li>
        <li>Not supported on all controllers</li>
      </ul></td>
  </tr>
</table>
<p>&nbsp;</p>
<h2>More information</h2>

<ul>
  <li><a href="craidint.htm">Understanding RAID technology</a> </li>
  <li><a href="cstripe.htm">Understanding stripe size</a> </li>
  <li><a href="cselraid.htm">Selecting a RAID level</a> </li>
  <li><a href="craid0.htm">RAID level-0</a> </li>
  <li><a href="craid1.htm">RAID level-1</a> </li>
  <li><a href="craid1e.htm">RAID level-1 Enhanced</a> </li>
  <li><a href="craid5.htm">RAID level-5</a> </li>
  <li><a href="craid5ee.htm">RAID level-5E Enhanced</a> </li>
  <li><a href="craidx0.htm">RAID level-x0</a></li>
  <li><a href="craidvol.htm">RAID volumes</a></li>
</ul>

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