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Suse openSUSE 12.2 Manual

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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, pdush 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 pdush thread wakes up (in hundredths of a second).
/proc/sys/vm/dirty_expire_centisecs
Denes 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 pdush 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 pdush daemon settings endanger the data integrity.
Apart from these processes, journaling le systems, like ReiserFS, Ext3, Ext4 and
others write their metadata independently from pdush, which also prevents the hard
disk from spinning down. To avoid this, a special kernel extension has been developed
for mobile devices. To make use of the extension, install the laptop-mode-tools
package and see /usr/src/linux/Documentation/laptops/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 modied le to the hard disk, causing
the disk to wake up. Features like this can be disabled at the expense of data integrity.
426 Reference
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