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What exactly is this setting?

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I was messing around looking for ways to conserve power and I came across this setting inside of "Disks". I've never heard about APM before but I suspect it has something to do with the HDDs speed.

So what exactly does this setting do? And does it really improve performance/save power?

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2 Answers 2

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Advanced Power Management (APM) is a BIOS API (functions) that the Operating system may use to implement multiple strategies for power management. For instance, it can be used to suspend or put in stand-by your computer.

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    but what does it do to the HDD?
    – Thunder_Ruler
    Sep 12, 2017 at 1:03
  • Old hard disks, after stop them, requires too much time to start again. Modern hard-disks can spin-down (reduce speed) after some idle time to reduce power consumption and then spin-up and achieve normal operation very fast. -- APM allows you to change the idle time before spin down if your hard disk is compatible. You may check askubuntu.com/questions/39760/…
    – Jaime
    Sep 12, 2017 at 2:09
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    okay I understand that but how come there are multiple levels of which either allow or disallow Spin down? Is it simply controlling the disk speed RPM without turning it down all the way?
    – Thunder_Ruler
    Sep 12, 2017 at 20:19
  • hdparam allows you set "levels": A low value means aggressive power management, i.e. less idle time before spin down and more frequent disk turn-offs. A high value means better performance, i.e. more idle time before spin down and less frequent turn-offs. -- The 255th level disables the spin-down. Your disk will be functioning normally all the time.
    – Jaime
    Sep 12, 2017 at 21:32
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Take a look at the manual page for hdparm. In particular the -B switch:

   -B     Get/set Advanced Power Management feature, if the drive supports
          it.  A  low  value  means aggressive power management and a high
          value means better performance.  Possible  settings  range  from
          values  1  through  127 (which permit spin-down), and values 128
          through 254 (which do not permit spin-down).  The highest degree
          of  power  management  is  attained with a setting of 1, and the
          highest I/O performance with a setting of 254.  A value  of  255
          tells  hdparm to disable Advanced Power Management altogether on
          the drive (not all drives support disabling it, but most do).
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