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<title>Understanding RAID level-1</title>

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<h1>Understanding RAID level-1 </h1>
<p>RAID level-1 uses data mirroring. Two physical drives are combined into an
  pool, and data is striped across the array. The first half of a stripe is the
  original data; the second half of a stripe is a <b>mirror</b> (that is, a copy)
  of the data, but it is written to the other drive in the RAID level-1 pool.
</p>
<p>RAID level-1 provides data redundancy and high levels of performance, but the
  storage capacity is diminished. Because the data is mirrored, the capacity of
  the logical device when assigned RAID level-1 is 50% of the pool capacity. </p>
<p>RAID level-1 requires two physical drives.
<p> The following illustration shows an example of a RAID level-1 logical device.
<h2>RAID level-1 example</h2>
<table border="0" cellpadding="3">
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="296">Start with two physical drives.
    </td>
    <td width="117"><img src="images/raidl0-a.gif" width="107" height="93"> </td>
  </tr>
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="296">Create an pool using the two physical
      drives. </td>
    <td width="117"><img src="images/raidl0-b.gif" width="107" height="93"> </td>
  </tr>
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="296">Then create a logical device within
      that pool. </td>
    <td width="117"><img src="images/raidl0-c.gif" width="107" height="93"> </td>
  </tr>
  <tr valign=top>
    <td align="LEFT" valign="TOP" width="296">The data is striped across the drives,
      creating blocks.
      <p>Notice that the data on the drive on the right is a copy of the data
        on the drive on the left.
    </td>
    <td width="117"><img src="images/raidl1-d.gif" width="107" height="93"> </td>
  </tr>
</table>
<p>With RAID level-1, if one of the physical drives fails, the controller
  switches read and write requests to the remaining functional drive in the RAID
  level-1 pool. </p>
<p>RAID level-1 offers the following advantages and disadvantages. <br>
</p>
<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 redundancy </li>
        <li>High performance </li>
      </ul>
      <br>
    </td>
    <td align="LEFT" valign="TOP" width="50%">Allows only 50% of the physical
      drive storage capacity to be used </td>
  </tr>
</table>
<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="craid5.htm">RAID level-5</a></li>
  <li><a href="craid6.htm">RAID level-6</a></li>
  <li> <a href="craidx0.htm">RAID level-x0</a> </li>
  <li><a href="craidvol.htm">RAID volumes</a></li>
  </li>
</ul>

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