Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 368 — #394

17.4 Rest for the Hard Disk

In Linux, a hard disk that is not used can be put to sleep. The hdparm util-

ity modiﬁes various hard disk settings. The option -y instantly switches

the hard disk to the standby mode. -Y (caution) puts it to sleep. hdparm -S

<x> causes the hard disk to be spun down after a certain period of inactiv-

ity. The placeholder hxi can be used as follows: 0 disables this mechanism,

causing the hard disk to run continuously. Values from 1 to 240 are multi-

plied by ﬁve seconds. Values from 241 to 251 correspond to one to eleven

times thirty minutes.

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 moni-

tored 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 ﬁve seconds and notiﬁes the bdﬂush daemon when data is older

than thirty seconds or the buffer reaches a ﬁll level of thirty percent. 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. On a stable

system, these settings can be modiﬁed. However, do not forget that this

may have a detrimental effect on the data integrity.

Caution

Impairment of the Data Integrity

Changes to the kernel update daemon settings affect the data

integrity. Do not touch these settings if you are not sure.

Caution

Specify the settings for the hard disk time-out, the kupdated interval,

the buffer threshold, and the age limit for data in /etc/sysconfig/

powermanagement for battery operation and for AC operation. The

variables are described in Section 17.2.1 on page 361 and in the ﬁle it-

self. Further information is available in /usr/share/doc/packages/

powersave.

368 17.4. Rest for the Hard Disk