Re: [tlug] shutting off the HD on a laptop

[Tobias Diedrich <ranma@example.com>]

Stephen J. Turnbull wrote:

> There's also a daemon called flushd or something like that that can
> inhibit flushes for a relatively long period of time.

noflushd IIRC, but AFAIK that one is obsoleted by the introduction of
/proc/sys/vm/laptop_mode in recent 2.6-kernels.

>From Documentation/laptop_mode.txt:

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How to conserve battery power using laptop-mode
-----------------------------------------------

Document Author: Bart Samwel (bart@example.com)
Date created: January 2, 2004
Last modified: July 10, 2004

Introduction
------------

Laptop mode is used to minimize the time that the hard disk needs to be spun up,
to conserve battery power on laptops. It has been reported to cause significant
power savings.

Contents
--------

* Introduction
* Installation
* Caveats
* The Details
* Tips & Tricks
* Control script
* ACPI integration
* Monitoring tool


Installation
------------

To use laptop mode, you don't need to set any kernel configuration options
or anything. Simply install all the files included in this document, and
laptop mode will automatically be started when you're on battery. For
your convenience, a tarball containing an installer can be downloaded at:

http://www.xs4all.nl/~bsamwel/laptop_mode/tools

To configure laptop mode, you need to edit the configuration file, which is
located in /etc/default/laptop-mode on Debian-based systems, or in
/etc/sysconfig/laptop-mode on other systems.

Unfortunately, automatic enabling of laptop mode does not work for
laptops that don't have ACPI. On those laptops, you need to start laptop
mode manually. To start laptop mode, run "laptop_mode start", and to
stop it, run "laptop_mode stop". (Note: The laptop mode tools package now
has experimental support for APM, you might want to try that first.)


Caveats
-------

* The downside of laptop mode is that you have a chance of losing up to 10
  minutes of work. If you cannot afford this, don't use it! The supplied ACPI
  scripts automatically turn off laptop mode when the battery almost runs out,
  so that you won't lose any data at the end of your battery life.

* Most desktop hard drives have a very limited lifetime measured in spindown
  cycles, typically about 50.000 times (it's usually listed on the spec sheet).
  Check your drive's rating, and don't wear down your drive's lifetime if you
  don't need to.

* If you mount some of your ext3/reiserfs filesystems with the -n option, then
  the control script will not be able to remount them correctly. You must set
  DO_REMOUNTS=0 in the control script, otherwise it will remount them with the
  wrong options -- or it will fail because it cannot write to /etc/mtab.

* If you have your filesystems listed as type "auto" in fstab, like I did, then
  the control script will not recognize them as filesystems that need remounting.
  You must list the filesystems with their true type instead.

* It has been reported that some versions of the mutt mail client use file access
  times to determine whether a folder contains new mail. If you use mutt and
  experience this, you must disable the noatime remounting by setting the option
  DO_REMOUNT_NOATIME to 0 in the configuration file.


The Details
-----------

Laptop mode is controlled by the knob /proc/sys/vm/laptop_mode. This knob is
present for all kernels that have the laptop mode patch, regardless of any
configuration options. When the knob is set, any physical disk I/O (that might
have caused the hard disk to spin up) causes Linux to flush all dirty blocks. The
result of this is that after a disk has spun down, it will not be spun up
anymore to write dirty blocks, because those blocks had already been written
immediately after the most recent read operation. The value of the laptop_mode
knob determines the time between the occurrence of disk I/O and when the flush
is triggered. A sensible value for the knob is 5 seconds. Setting the knob to
0 disables laptop mode.

To increase the effectiveness of the laptop_mode strategy, the laptop_mode
control script increases dirty_expire_centisecs and dirty_writeback_centisecs in
/proc/sys/vm to about 10 minutes (by default), which means that pages that are
dirtied are not forced to be written to disk as often. The control script also
changes the dirty background ratio, so that background writeback of dirty pages
is not done anymore. Combined with a higher commit value (also 10 minutes) for
ext3 or ReiserFS filesystems (also done automatically by the control script),
this results in concentration of disk activity in a small time interval which
occurs only once every 10 minutes, or whenever the disk is forced to spin up by
a cache miss. The disk can then be spun down in the periods of inactivity.

If you want to find out which process caused the disk to spin up, you can
gather information by setting the flag /proc/sys/vm/block_dump. When this flag
is set, Linux reports all disk read and write operations that take place, and
all block dirtyings done to files. This makes it possible to debug why a disk
needs to spin up, and to increase battery life even more. The output of
block_dump is written to the kernel output, and it can be retrieved using
"dmesg". When you use block_dump and your kernel logging level also includes
kernel debugging messages, you probably want to turn off klogd, otherwise
the output of block_dump will be logged, causing disk activity that is not
normally there.


Configuration
-------------

The laptop mode configuration file is located in /etc/default/laptop-mode on
Debian-based systems, or in /etc/sysconfig/laptop-mode on other systems. It
contains the following options:

MAX_AGE:

Maximum time, in seconds, of hard drive spindown time that you are
confortable with. Worst case, it's possible that you could lose this
amount of work if your battery fails while you're in laptop mode.

MINIMUM_BATTERY_MINUTES:

Automatically disable laptop mode if the remaining number of minutes of
battery power is less than this value. Default is 10 minutes.

AC_HD/BATT_HD:

The idle timeout that should be set on your hard drive when laptop mode
is active (BATT_HD) and when it is not active (AC_HD). The defaults are
20 seconds (value 4) for BATT_HD  and 2 hours (value 244) for AC_HD. The
possible values are those listed in the manual page for "hdparm" for the
"-S" option.

HD:

The devices for which the spindown timeout should be adjusted by laptop mode.
Default is /dev/hda. If you specify multiple devices, separate them by a space.

READAHEAD:

Disk readahead, in 512-byte sectors, while laptop mode is active. A large
readahead can prevent disk accesses for things like executable pages (which are
loaded on demand while the application executes) and sequentially accessed data
(MP3s).

DO_REMOUNTS:

The control script automatically remounts any mounted journaled filesystems
with approriate commit interval options. When this option is set to 0, this
feature is disabled.

DO_REMOUNT_NOATIME:

When remounting, should the filesystems be remounted with the noatime option?
Normally, this is set to "1" (enabled), but there may be programs that require
access time recording.

DIRTY_RATIO:

The percentage of memory that is allowed to contain "dirty" or unsaved data
before a writeback is forced, while laptop mode is active. Corresponds to
the /proc/sys/vm/dirty_ratio sysctl.

DIRTY_BACKGROUND_RATIO:

The percentage of memory that is allowed to contain "dirty" or unsaved data
after a forced writeback is done due to an exceeding of DIRTY_RATIO. Set
this nice and low. This corresponds to the /proc/sys/vm/dirty_background_ratio
sysctl.

Note that the behaviour of dirty_background_ratio is quite different
when laptop mode is active and when it isn't. When laptop mode is inactive,
dirty_background_ratio is the threshold percentage at which background writeouts
start taking place. When laptop mode is active, however, background writeouts
are disabled, and the dirty_background_ratio only determines how much writeback
is done when dirty_ratio is reached.

DO_CPU:

Enable CPU frequency scaling when in laptop mode. (Requires CPUFreq to be setup.
See Documentation/cpu-freq/user-guide.txt for more info. Disabled by default.)

CPU_MAXFREQ:

When on battery, what is the maximum CPU speed that the system should use? Legal
values are "slowest" for the slowest speed that your CPU is able to operate at,
or a value listed in /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies.


Tips & Tricks
-------------

* Bartek Kania reports getting up to 50 minutes of extra battery life (on top
  of his regular 3 to 3.5 hours) using a spindown time of 5 seconds (BATT_HD=1).

* You can spin down the disk while playing MP3, by setting disk readahead
  to 8MB (READAHEAD=16384). Effectively, the disk will read a complete MP3 at
  once, and will then spin down while the MP3 is playing. (Thanks to Bartek
  Kania.)

* Drew Scott Daniels observed: "I don't know why, but when I decrease the number
  of colours that my display uses it consumes less battery power. I've seen
  this on powerbooks too. I hope that this is a piece of information that
  might be useful to the Laptop Mode patch or it's users."

* In syslog.conf, you can prefix entries with a dash ``-'' to omit syncing the
  file after every logging. When you're using laptop-mode and your disk doesn't
  spin down, this is a likely culprit.

* Richard Atterer observed that laptop mode does not work well with noflushd
  (http://noflushd.sourceforge.net/), it seems that noflushd prevents laptop-mode
  from doing its thing.

* If you're worried about your data, you might want to consider using a USB
  memory stick or something like that as a "working area". (Be aware though
  that flash memory can only handle a limited number of writes, and overuse
  may wear out your memory stick pretty quickly. Do _not_ use journalling
  filesystems on flash memory sticks.)
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Above document further includes a shell script and some c code,
which I left out.

-- 
Tobias						PGP: http://9ac7e0bc.uguu.de
Mail over UUCP over Bicycle+Train