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. Most of the functions can be controlled with powersaved and the YaST power management module, which is discussed in further detail in Section 33.6, “The YaST Power Management Module”.
The hdparm application can be used to modify various hard disk settings. The
-y instantly switches the hard disk to the standby
-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
240 are multiplied by 5
seconds. Values from
correspond to 1 to 11 times 30 minutes.
Internal power saving options of the hard disk
can be controlled with the option
Select a value from
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
Select a value from
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 kernel update daemon (kupdated). 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, kupdated is set to short intervals to achieve maximum data integrity. It checks the buffer every 5 seconds and notifies the bdflush daemon when data is older than 30 seconds or the buffer reaches a fill level of 30%. The bdflush daemon then writes the data to the hard disk. It also writes independently from kupdated if, for instance, the buffer is full.
|Impairment of the Data Integrity|
Changes to the kernel update daemon settings endanger the data integrity.
Apart from these processes, journaling file systems, like ReiserFS and Ext3,
write their metadata independently from bdflush, which also prevents the
hard disk from spinning down. To avoid this, a special kernel extension has
been developed for mobile devices. See
Another important factor is the way active programs behave. For example, good editors regularly write hidden backups of the currently modified file to the hard disk, causing the disk to wake up. Features like this can be disabled at the expense of data integrity.
In this connection, the mail daemon postfix makes use of the variable
POSTFIX_LAPTOP. If this variable is set to
yes, postfix accesses the hard disk far less
frequently. However, this is irrelevant if the interval for kupdated was