40 lines
1.1 KiB
Text
40 lines
1.1 KiB
Text
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.SH
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The read-ahead processes
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.PP
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There are a set of file system processes,
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.CW rah ,
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that wait for messages consisting of a device and block
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address.
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When a message comes in,
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the process reads the specified block from the device.
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This is done by calling
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.CW getbuf
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and
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.CW putbuf .
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The purpose of this is the hope that these blocks
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will be used later and that they will reside in the
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buffer cache long enough not to be discarded before
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they are used.
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.PP
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The messages to the read-ahead processes are
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generated by the server processes.
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The server processes maintain a relative block mark in every
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open file.
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Whenever an open file reads that relative block,
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the next 110 block addresses of the file are sent
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to the read-ahead processes and
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the relative block mark is advanced by 100.
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The initial relative block is set to 1.
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If the file is opened and
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only a few bytes are read,
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then no anticipating reads are performed
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since the relative block mark is set to 1
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and only block offset 0 is read.
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This is to prevent some
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fairly common action such as
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.Ex
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file *
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.Ee
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from swamping the file system with read-ahead
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requests that will never be used.
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