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Automatic Acoustic Management
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Most modern harddisk drives have the ability to speed down the head movements
to reduce their noise output. The possible values are between 0 and 254. 128
is the most quiet (and therefore slowest) setting and 254 the fastest (and loudest).
Some drives have only two levels (quiet / fast), while others may have different
levels realized between 128 and 254.

To be able to use this with hdparm, you will need a current kernel with the -ac patches
applied. Maybe the kernel of your favorite distribution will already include this (SuSE
has it in their current kernel, for example).
Just try to compile hdparm, type "hdparm" and have a look for "-M" in the output. If it 
doesn't appear, your kernel most likely doesn't support it. 

You can get the acoustic setting by typing

	hdparm -M /dev/hda 

To set the most quiet mode use

	hdparm -M 128 /dev/hda

For the fastest setting use

	hdparm -M 254 /dev/hda

Now test different values and try to hear the difference. :-) 

Not all disk drives support this setting - and the speed impacts may also vary between
different manufacturers and models.  

If you see lines like

	kernel: hdb: task_no_data_intr: status=0x51 { DriveReady SeekComplete Error }
	kernel: hdb: task_no_data_intr: error=0x04 { DriveStatusError }

in your syslog, then your harddisk will most likely not support Acoustic Management.

Perhaps you also wanna have a look on the IBM Feature Tool, available at 
http://www.storage.ibm.com/hdd/support/download.htm.

I would welcome any feedback to <gernot@hillier.de>

Gernot