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Top Forums UNIX for Dummies Questions & Answers Writing file to disk takes time Post 37265 by Perderabo on Monday 16th of June 2003 07:36:00 AM
Old 06-16-2003
Unix has a buffer cache which is a very large chunk of main memory.

When you issue a disk read, unix sees if the data is already in the buffer cache, if so no i/o occurs, the data is just moved into your program's buffer. When you issue a write, the data is just moved into the buffer cache.

The system periodicly scans the buffer cache and flushes new data to disk. This is called "write-behind". The system also predicts data that you are likely to need and issues reads to get the data into the buffer cache before your program wants it. This is called "read-ahead".

Read-ahead and write-behind work so well that most disk i/o is reduced to just moving data in memory.
 

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VFS_READAHEAD(8)					    System Administration tools 					  VFS_READAHEAD(8)

NAME
vfs_readahead - pre-load the kernel buffer cache SYNOPSIS
vfs objects = readahead DESCRIPTION
This VFS module is part of the samba(7) suite. This vfs_readahead VFS module detects read requests at multiples of a given offset (hex 0x80000 by default) and then tells the kernel via either the readahead system call (on Linux) or the posix_fadvise system call to pre-fetch this data into the buffer cache. This module is useful for Windows Vista clients reading data using the Windows Explorer program, which asynchronously does multiple file read requests at offset boundaries of 0x80000 bytes. The offset multiple used is given by the readahead:offset option, which defaults to 0x80000. The size of the disk read operations performed by vfs_readahead is determined by the readahead:length option. By default this is set to the same value as the readahead:offset option and if not set explicitly will use the current value of readahead:offset. This module is stackable. OPTIONS
readahead:offset = BYTES The offset multiple that causes readahead to be requested of the kernel buffer cache. readahead:length = BYTES The number of bytes requested to be read into the kernel buffer cache on each readahead call. The following suffixes may be applied to BYTES: o K - BYTES is a number of kilobytes o M - BYTES is a number of megabytes o G - BYTES is a number of gigabytes EXAMPLES
[hypothetical] vfs objects = readahead VERSION
This man page is correct for version 3.0.25 of the Samba suite. AUTHOR
The original Samba software and related utilities were created by Andrew Tridgell. Samba is now developed by the Samba Team as an Open Source project similar to the way the Linux kernel is developed. Samba 4.0 06/17/2014 VFS_READAHEAD(8)
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