Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual page 195

Values from 1 to 240 are multiplied by 5 seconds. Values from 241 to 251 correspond
to 1 to 11 times 30 minutes.
Internal power saving options of the hard disk can be controlled with the option -B.
Select a value from 0 to 255 for maximum saving to maximum throughput. The result
depends on the hard disk used and is difficult to assess. To make a hard disk quieter,
use the option -M. Select a value from 128 to 254 for quiet to fast.
Often, it is not so easy to put the hard disk to sleep. In Linux, numerous processes write
to the hard disk, waking it up repeatedly. Therefore, it is important to understand how
Linux handles data that needs to be written to the hard disk. First, all data is buffered
in the RAM. This buffer is monitored by the pdflush daemon. When the data reaches
a certain age limit or when the buffer is filled to a certain degree, the buffer content is
flushed to the hard disk. The buffer size is dynamic and depends on the size of the
memory and the system load. By default, pdflush is set to short intervals to achieve
maximum data integrity. It checks the buffer every 5 seconds and writes the data to the
hard disk. The following variables are interesting:
/proc/sys/vm/dirty_writeback_centisecs
Contains the delay until a pdflush thread wakes up (in hundreths of a second).
/proc/sys/vm/dirty_expire_centisecs
Defines after which timeframe a dirty page should be written out latest. Default is
3000, which means 30 seconds.
/proc/sys/vm/dirty_background_ratio
Maximum percentage of dirty pages until pdflush begins to write them. Default is
5%.
/proc/sys/vm/dirty_ratio
When the dirty page exceeds this percentage of the total memory, processes are
forced to write dirty buffers during their time slice instead of continuing to write.
WARNING: Impairment of the Data Integrity
Changes to the pdflush daemon settings endanger the data integrity.
Apart from these processes, journaling file systems, like ReiserFS and Ext3, write their
metadata independently from pdflush, which also prevents the hard disk from spinning
Power Management 181