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<p>Use raw devices&#047;volumes for Oracle files only if your site has at least as many raw disk partitions as Oracle datafiles. If the raw disk partitions are already formatted, match datafile size to the partition size as closely as possible to avoid wasting space. In a heavily-used database configuration, use raw devices.
</p>
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<p>
<font face="arial, helvetica, sans-serif">
<strong>Note:</strong>
</font>
</p>Oracle9<em>i</em> Real Application Clusters requires the use of raw devices instead of a journaled file system for database files.
</td>
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</table><br />
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<div class="sect3"><a id="sthref642" name="sthref642"></a>
<h4>
<font face="arial, helvetica, sans-serif" color="#330099">
Moving from a Journaled File System to Raw Devices
</font>
</h4>
<p>To move from a journaled file system to raw devices without the need to manually reload all of the data, enter the following command as the <code>root</code> user:
</p>
<pre>&#035; dd if&#061;<em>old&#095;JFS&#095;file</em> of&#061;<em>new&#095;raw&#095;device</em> seek&#061;1 bs&#061;4k 

</pre>
<p>Ensure that you follow these guidelines:
</p>
<ul>
<li type="disc">
<p>Skip the first 4 KB block of the raw device because the AIX Logical Volume Manager uses it.
</p>
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<p>
<font face="arial, helvetica, sans-serif">
<strong>Note:</strong>
</font>
</p>The first 4 KB block should not be skipped if the raw device is a Hashed Shared Disk (HSD).
</td>
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</table><br />
</li>
<li type="disc">
<p>Size the raw device to prevent wasting space. The raw device should be a few blocks larger than the existing JFS or JFS2 file.
</p>
</li>
<li type="disc">
<p>Set the permissions on the raw partition.
</p>
</li>
<li type="disc">
<p>Rename the datafile.
</p>
</li>
</ul>
<p>You might experience a slight degradation in sequential read performance. If the application is performing many full table scans, you might experience a server performance degradation on these full table scans, because the AIX file buffer cache is no longer used to cache these tables.
</p>
<p>In configurations where memory is scarce, you might need to resize the AIX file buffer cache.
</p>
<p>In the AIX environment, Oracle Corporation encourages the use of symbolic links whenever possible, because the name of the raw device can change. For example, this change can occur when reconfiguring disk peripherals or moving the database to a new hardware configuration. Refer to the IBM documentation for more information.
</p>
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<p>
<font face="arial, helvetica, sans-serif">
<strong>See Also:</strong>
</font>
</p><em>Oracle9i Installation Guide Release 2 (9.2.0.1.0) for UNIX Systems</em> for more information on the operational issues involved in using raw devices.
</td>
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</table><br />
</div>
</div class="sect3">
<div class="sect3"><a id="sthref643" name="sthref643"></a>
<h4>
<font face="arial, helvetica, sans-serif" color="#330099">
Taking Advantage of Both Journaled File Systems and Raw Devices
</font>
</h4>
<p>On AIX, you can choose a journaled file system for some datafiles and raw partitions for other datafiles. Asynchronous I&#047;O works with both raw partitions and a journaled file system. If you know the nature of I&#047;O activity on database objects beforehand, you can place the datafiles corresponding to specific objects on either a journaled file system or raw partitions, together with an LVM, if available. 
</p>
<p>The improvement of performance varies greatly depending on the nature of your workload and the disk or file system configuration.
</p>
</div class="sect3">
</div class="sect2"><a id="i631413" name="i631413"></a>
<div class="sect2"><a id="sthref644" name="sthref644"></a>
<h3>
<font face="arial, helvetica, sans-serif" color="#330099">
Using Asynchronous I&#047;O<a id="sthref645" name="sthref645"></a>
</font>
</h3>
<p>Oracle9<em>i</em> takes full advantage of asynchronous I&#047;O (AIO) provided by AIX, resulting in faster database access. AIO interweaves multiple I&#047;O to improve I&#047;O subsystem throughput. The advantage of AIO is realized only when data is well distributed among different disks. 
</p>
<p>Using the LVM and striping enhances the effectiveness of AIO. The LVM reduces disk contention by striping data across multiple disk spindles. Using AIO with LVM significantly improves RDBMS performance.
</p>
<p>AIX versions 4 and higher support asynchronous I&#047;O (AIO) for database files created both on file system partitions and on raw devices. AIO on raw devices is implemented fully into the AIX kernel, and does not require server processes to service the AIO requests. When using AIO on file systems, the kernel server processes (<code>kproc</code>) control each request from the time a request is taken off the queue until it completes. The kernel server processes are also used with I&#047;O with virtual shared disks (VSDs) and HSDs with FastPath disabled. By default, FastPath is enabled. The number of <code>kproc</code> servers determines the number of AIO requests that can be executed in the system concurrently, so it is important to tune the number of <code>kproc</code> processes when using filesystems to store Oracle9<em>i</em> datafiles.
</p>
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<font face="arial, helvetica, sans-serif">
<strong>Note:</strong>
</font>
</p>If you are using AIO with VSDs and HSDs with AIO FastPath enabled (the default), the maximum buddy buffer size must be greater than or equal to 128 KB.
</td>
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</table><br />
</div>
<p>Use one of the following commands to set the number of servers:
</p>
<ul>
<li type="disc">
<p><code>smit aio</code>
</p>
</li>
<li type="disc">
<p><code>chdev -l aio0 -a maxservers&#061;&#039;</code><em>m</em><code>&#039; -a minservers&#061;&#039;</code><em>n</em><code>&#039; </code>
</p>
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<p>
<font face="arial, helvetica, sans-serif">
<strong>See Also:</strong>
</font>
</p>System Management Interface Tool (SMIT) online help for information on SMIT, and the man pages for information on the <code>smit</code> <code>aio</code> and <code>chdev</code> commands.
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</table><br />
</li>
</ul>
<p>Set the minimum value to the number of servers to be started at system boot. Set the maximum value to the number of servers that can be started in response to a large number of concurrent requests. These parameters apply to files only, they do not apply to raw devices. 
</p>
<p>The default value for the minimum number of servers is 1. The default value for the maximum number of servers is 10. These values are usually too low to run the Oracle server if you are not using kernelized AIO. Oracle Corporation recommends that you set the following values:
</p>
<table title="MAXSERVERS and MINSERVERS Parameter Values" summary="This table lists the recommended values for the MAXSERVERS and MINSERVERS parameters." dir="ltr" border="1" width="100%" frame="hsides" rules="groups" cellpadding="3" cellspacing="0">
<thead>
<tr align="left" valign="top">
<th id="r1c1" align="left" colspan="1" rowspan="1" valign="bottom">
<font face="Arial, Helvetica, sans-serif">
<strong>Parameter
</strong></font></th>
<th id="r1c2" align="left" colspan="1" rowspan="1" valign="bottom">
<font face="Arial, Helvetica, sans-serif">
<strong>Value
</strong></font></th>
</tr>
</thead><tbody>
<tr align="left" valign="top">
<td id="r2c1" headers="r1c1" align="left" colspan="1" rowspan="1">MINSERVERS
</td>
<td headers="r2c1 r1c2" align="left" colspan="1" rowspan="1">Varies depending on the asynchronous requests to the AIO servers on the system. Oracle Corporation recommends an initial value of 10.
</td>
</tr>
<tr align="left" valign="top">
<td id="r3c1" headers="r1c1" align="left" colspan="1" rowspan="1">MAXSERVERS
</td>
<td headers="r3c1 r1c2" align="left" colspan="1" rowspan="1">You can incrementally increase this value to a maximum of the value of the MAXSERVERS parameter divided by 2 or the number of disks accessed divided by 10, whichever is greater.
</td>
</tr>
<tr align="left" valign="top">
<td id="r4c1" headers="r1c1" align="left" colspan="1" rowspan="1">MAXREQS
</td>
<td headers="r4c1 r1c2" align="left" colspan="1" rowspan="1">8192
</td>
</tr></tbody>
</table>
<p>If the value of the MAXSERVERS parameter is set too low, you might see the following error messages repeated:
</p>
<pre>Warning: lio&#095;listio returned EAGAINPerformance degradation may be seen.

</pre>
<p>You can avoid these errors by increasing the value of the MAXSERVERS parameter to greater than the number of AIO servers running. To display the number of AIO servers running, enter the following command as the <code>root</code> user:
</p>
<pre>&#035; pstat -a &#124; grep -c aios

</pre>
<p>Check the number of active AIO servers periodically and change the values of the MINSERVERS and MAXSERVERS parameters if necessary. The changes take place when the system restarts.
</p>
</div class="sect2">
<div class="sect2"><a id="sthref646" name="sthref646"></a>
<h3>
<font face="arial, helvetica, sans-serif" color="#330099">
I&#047;O Slaves<a id="sthref647" name="sthref647"></a><a id="sthref648" name="sthref648"></a>
</font>
</h3>
<p>I&#047;O Slaves are specialized processes that perform only I&#047;O. They can operate whether or not asynchronous I&#047;O is available. They are allocated from shared memory buffers. I&#047;O Slaves use a set of initialization parameters, listed in the following table, that allow a degree of control over the way they operate.
</p>
<table title="I/O Slave Parameter Table" summary="This table lists the I/O slave paramters, their range and default values." dir="ltr" border="1" width="100%" frame="hsides" rules="groups" cellpadding="3" cellspacing="0">
<thead>
<tr align="left" valign="top">
<th id="r1c1" align="left" colspan="1" rowspan="1" valign="bottom">
<font face="Arial, Helvetica, sans-serif">
<strong>Parameter
</strong></font></th>
<th id="r1c2" align="left" colspan="1" rowspan="1" valign="bottom">
<font face="Arial, Helvetica, sans-serif">
<strong>Range of Values
</strong></font></th>
<th id="r1c3" align="left" colspan="1" rowspan="1" valign="bottom">
<font face="Arial, Helvetica, sans-serif">
<strong>Default Value
</strong></font></th>
</tr>
</thead><tbody>
<tr align="left" valign="top">
<td id="r2c1" headers="r1c1" align="left" colspan="1" rowspan="1">DISK&#095;ASYNCH&#095;IO
</td>
<td headers="r2c1 r1c2" align="left" colspan="1" rowspan="1">TRUE&#047;FALSE
</td>
<td headers="r2c1 r1c3" align="left" colspan="1" rowspan="1">TRUE
</td>
</tr>
<tr align="left" valign="top">
<td id="r3c1" headers="r1c1" align="left" colspan="1" rowspan="1">TAPE&#095;ASYNCH&#095;IO
</td>
<td headers="r3c1 r1c2" align="left" colspan="1" rowspan="1">TRUE&#047;FALSE
</td>
<td headers="r3c1 r1c3" align="left" colspan="1" rowspan="1">TRUE
</td>
</tr>
<tr align="left" valign="top">
<td id="r4c1" headers="r1c1" align="left" colspan="1" rowspan="1">BACKUP&#095;TAPE&#095;IO&#095;SLAVES
</td>
<td headers="r4c1 r1c2" align="left" colspan="1" rowspan="1">TRUE&#047;FALSE
</td>
<td headers="r4c1 r1c3" align="left" colspan="1" rowspan="1">FALSE
</td>
</tr>
<tr align="left" valign="top">
<td id="r5c1" headers="r1c1" align="left" colspan="1" rowspan="1">DBWR&#095;IO&#095;SLAVES