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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 inde-
pendently from kupdated if, for instance, the buffer is full.
WARNING: 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 /usr/src/linux/Documentation/laptop-mode.txt for details.
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 increased.
28.5 The powersave Package
The powersave package cares about all the previously-mentioned power saving
functions. Due to the increasing demand for lower energy consumption in general, some
of its features are also important on workstations and servers, such as suspend, standby,
or CPU frequency scaling.
This package contains all power management features of your computer. It supports
hardware using ACPI, APM, IDE hard disks, and PowerNow! or SpeedStep technologies.
The functions from the packages apmd, acpid, ospmd, and cpufreqd (now
cpuspeed) have been consolidated in the powersave package. Daemons from these
Power Management
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Troubleshooting
