Run -time cache settings

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.

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.

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 -minimising the overhead of metadata updates that the file system performs.

Opening files with different modes

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:

ON
ENABLED
OFF

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 TFileMode enumeration supports the run time file caching -configurations. The file open modes are as follows:

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

If the -values above are not specified when a file is opened then these default settings -are used:

if caching is set to ENABLED for a drive by the partner then caching -is OFF by default
if caching is set to ON for a drive by the partner then caching is -ON by default

If caching is set to OFF for a particular drive by the partner, -caching cannot be enabled.

Use the TFileMode values -with the following functions to alter the way in which an open file is accessed.

RFile::Open()
RFile::Create()
RFile::Replace()
RFile::Temp()