12.2.4 Filesystem I/O
Reading and writing of data on disk file systems is usually buffered. Data are not actually read from disk for every read request made by a user, and they are not written to disk for every write request made by a user. Data pass through buffers on their way to and from disks. When a user program opens and reads from a file, more data are actually read from disk than were requested, and the extra data are stored in a buffer. Subsequent read requests are fulfilled from the buffer until the buffer is empty. Then, the buffer is filled by another read from the disk. When writing to files, data are written into a buffer until the buffer fills, and then, the contents of the buffer is written to disk.
How long will data sit in an output buffer if only a small amount of data are written to a file and the buffer is very slow to fill? Most Unix systems run a daemon that regularly flushes buffers. Flush times usually vary from 15 seconds to 1 minute. People who run Unix on laptops often set very long flush times to minimize disk writes and power consumption to prolong battery life.
Ordinary users can flush buffers by using the sync command. Buffers are also flushed when filesystems are unmounted using the umount command.
http://www.ussg.iu.edu/UAU/filesystem/physical.html
The modification of the filesystem cache-tuning parameters can significantly improve Linux's file-serving performance up to a factor of two. Linux will attempt to use memory not being used for any other purpose for filesystem caching. A special daemon, called bdflush, will periodically flush dirty buffers, buffers that contain modified filesystem data or metadata to the disk.
http://tldp.org/LDP/solrhe/Securing-...p29sec287.html