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<title>Understanding write-cache mode for logical devices</title>

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<h1>Understanding write-cache mode for logical devices </h1>
<p>When using the write-cache-mode option, you can choose from two available settings.</p>
<h2>Write back</h2>
<p>For the write-back setting, the operating system sends data to the controller
  to write to a storage device. Subsequently, the controller sends a confirmation
  to the operating system <b>before</b> actually writing the data to the storage
  device. Doing so increases performance, but also contains an element of risk.
  For example, if there is a power failure, the data currently in the controller
  cache is lost. This is no risk when using a controller with a <a href="gbbc.htm">battery-backup
  cache</a>. The battery preserves the data in the controller cache in the event
  of a power failure. </p>
<dl>
  <dt><b><img src="images/attentio.gif" width="22" height="15">Attention</b>:</dt>
  <dd>
    <ol>
      <li> It is possible to lose data if a power outage occurs while using the
        write back setting without a battery-backup cache device. If your controller
        does not have a battery-backup cache installed and enabled, consider carefully
        whether to enable write back on a logical device. Depending on how you
        use the system, write back might be undesirable. <br>
        <br>
      </li>
      <li> If you do not have a battery-backup cache installed and enabled and
        you set this feature to write back, wait at least 10 seconds after your
        last operation before you turn off your system. Failure to follow this
        practice might result in lost data. </li>
    </ol>
  </dd>
</dl>
<h2>Write through</h2>
<p>For the write-through<b> </b>setting, the operating system sends data to the
  controller to write to a storage device. Subsequently, the controller writes
  the data to the storage device; then, sends a confirmation to the operating
  system. This setting can decrease performance, but contains no risk of losing
  data.</p>
<h2>More information</h2>
<ul>
  <li><a href="awriteld.htm">Changing the write-cache mode on a logical device
    (action)</a></li>
  <li><a href="cfintune.htm">Fine-tuning your system</a></li>
</ul>

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