APM_level的全稱為Advanced Power Management level,即(硬盤)高級電源管理級別.

man hdparm 可見:
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).

該值的取值范圍為1~255,值越大,硬盤性能越高,其中255表示關閉硬盤/dev/sda的電源管理,254表示在開啟電源管理下的最高性能,1表示最低的硬盤性能但最省電.
這個值在1~127區間時允許spin-down,在128~254這個區間則禁止spin-down.
spin-down是指在閑置時掛起硬盤,停止轉動,但是頻繁的spin-down和spin-up會使硬盤壽命變短.

所以,即使你追求節能,減少發熱量,也建議你將該值設為128,執行以下命令即可:

注意: 設置APM后執行以下命令查看Load_Cycle_Count值