19.3 Rest for the Hard Disk
In Linux, the hard disk can be put to sleep entirely if it is not needed or it can be run in
a more economic or quieter mode. On modern laptops, you do not need to switch off
the hard disks manually, because they automatically enter an economic operating mode
whenever they are not needed. However, if you want to maximize power savings, test
some of the following methods, using the hdparm command.
It can be used to modify various hard disk settings. The option -y instantly switches
the hard disk to the standby mode. -Y puts it to sleep. hdparm -S x causes the hard
disk to be spun down after a certain period of inactivity. Replace x as follows: 0 disables
this mechanism, causing the hard disk to run continuously. 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 difcult 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 lled to a certain degree, the buffer content is
ushed to the hard disk. The buffer size is dynamic and depends on the size of the
memory and the system load. By default, pdush 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 pdush thread wakes up (in hundredths of a second).
/proc/sys/vm/dirty_expire_centisecs
Denes after which timeframe a dirty page should be written out latest. Default is
3000, which means 30 seconds.
356 Reference
