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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

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

    13 time cache settings</title><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 

    15 

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

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

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

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

    20 the data from the media. </p>

    21 <p> <b>Note</b>: Read caching has little benefit when files are accessed sequentially

    22 in large blocks, and are never re-read. </p>

    23 

    24 

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

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

    27 performing streaming operations and speculatively reading ahead on the assumption

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

    29 future, thus improving performance. </p>

    30 

    31 

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

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

    34 that overcomes the inefficiencies of clients that perform small write operations

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

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

    37 minimising the overhead of metadata updates that the file system performs. </p>

    38 <section><title>Opening files with different modes</title> <p>The system is

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

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

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

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

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

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

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

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

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

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

    49 EFileWriteDirectIO  =0x00001000, // Disables write buffering

    50 EFileReadBuffered   =0x00002000, // Enables read buffering

    51 EFileReadDirectIO   =0x00004000, // Disables read buffering

    52 EFileReadAheadOn    =0x00008000, // Enables read ahead 

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

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

    55 are used:</p><ul>

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

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

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

    59 ON by default</p></li>

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

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

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

    63 <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>

    64 <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>

    65 <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>

    66 <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>

    67 </ul></section>

    68 </conbody></concept>