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<concept id="GUID-3FEEFC4D-19B5-502A-8310-40646B9C34BC" xml:lang="en"><title>Run

time cache settings</title><shortdesc>This topic describes the rum time cache settings.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p><b>Read caching </b> </p>

<p>A client, or multiple clients, may issue repeated requests to read data

from the same locality within a file. Data that has been read previously that

is still in the cache can be returned to the client without continuously re-reading

the data from the media. <note>Read caching has little benefit when files

are accessed sequentially in large blocks, and are never re-read. </note> </p>

<p><b>Read ahead caching </b> </p>

<p>Read ahead caching builds on read caching by detecting clients that are

performing streaming operations and speculatively reading ahead on the assumption

that, once the data is in the cache it is likely to be accessed in the near

future, thus improving performance. </p>

<p><b>Write caching </b> </p>

<p>Write caching is implemented to perform a small level of write back caching

that overcomes the inefficiencies of clients that perform small write operations

by consolidating multiple file updates into a single larger write operation,

thus taking advantage of media that is written on a block basis as well as

minimizing the overhead of metadata updates that the file system

performs. </p>

<section id="GUID-8653BC99-0FA6-4FA0-AEBD-066C0BE91B57"><title>Opening files with different modes</title> <p>The system is

configured by partners. Read caching, read ahead caching and write caching

are set on a per drive basis and are one of three states, these are:</p><ul>

<li><p>ON </p></li>

<li><p>ENABLED </p></li>

<li><p>OFF</p></li>

</ul><p>If caching is set to ENABLED or ON for a drive by the partner then

you, as the client, can enable or disable caching at run-time on a per file

basis. However, if caching is set to OFF it cannot be altered at run-time.

The <codeph>TFileMode</codeph> enumeration supports the run time file caching

configurations. The file open modes are as follows: </p><codeblock xml:space="preserve">EFileWriteBuffered  =0x00000800, // Enables write buffering

EFileWriteDirectIO  =0x00001000, // Disables write buffering

EFileReadBuffered   =0x00002000, // Enables read buffering

EFileReadDirectIO   =0x00004000, // Disables read buffering

EFileReadAheadOn    =0x00008000, // Enables read ahead 

EFileReadAheadOff   =0x00010000  // Disables read ahead</codeblock><p>If the

values above are not specified when a file is opened then these default settings

are used:</p><ul>

<li><p>if caching is set to ENABLED for a drive by the partner then caching

is OFF by default </p></li>

<li><p>if caching is set to ON for a drive by the partner then caching is

ON by default</p></li>

</ul><note>If caching is set to OFF for a particular drive by the partner,

caching cannot be enabled.</note><p>Use the <codeph>TFileMode</codeph> values

with the following functions to alter the way in which an open file is accessed. </p><ul>

<li><p><xref href="GUID-BE0804F6-4375-3C8A-8C83-968F510466E0.dita#GUID-BE0804F6-4375-3C8A-8C83-968F510466E0/GUID-72470A68-7E07-30EF-A3C8-AA855CDAF60E"><apiname>RFile::Open()</apiname></xref></p></li>

<li><p><xref href="GUID-BE0804F6-4375-3C8A-8C83-968F510466E0.dita#GUID-BE0804F6-4375-3C8A-8C83-968F510466E0/GUID-1EB4AA8F-9363-3EC5-8AE2-1F70F15FCF15"><apiname>RFile::Create()</apiname></xref></p></li>

<li><p><xref href="GUID-BE0804F6-4375-3C8A-8C83-968F510466E0.dita#GUID-BE0804F6-4375-3C8A-8C83-968F510466E0/GUID-C2069B9B-04C0-3878-BF08-289DBC83DDF8"><apiname>RFile::Replace()</apiname></xref></p></li>

<li><p><xref href="GUID-BE0804F6-4375-3C8A-8C83-968F510466E0.dita#GUID-BE0804F6-4375-3C8A-8C83-968F510466E0/GUID-87D0C77C-CC63-3D70-BACE-3C5EE702BD37"><apiname>RFile::Temp()</apiname></xref></p></li>

</ul></section>