Aggressive Link Power Management (ALPM) is a power-saving technique that helps the disk save power by setting a SATA link to the disk to a low-power setting during idle time (that is when there is no I/O). ALPM automatically sets the SATA link back to an active power state once I/O requests are queued to that link.

Power savings introduced by ALPM come at the expense of disk latency. As such, you should only use ALPM if you expect the system to experience long periods of idle I/O time.

ALPM is only available on SATA controllers that use the Advanced Host Controller Interface (AHCI). For more information about AHCI, refer to http://www.intel.com/technology/serialata/ahci.htm.

When available, ALPM is enabled by default. ALPM has three modes:

medium_power mode allows the link to transition between PARTIAL and fully-powered (that is "ACTIVE") states, depending on the load. Note that it is not possible to transition a link directly from PARTIAL to SLUMBER and back; in this case, either power state cannot transition to the other without transitioning through the ACTIVE state first.

To check whether your SATA host adapters actually support ALPM you can check if the file /sys/class/scsi_host/host*/link_power_management_policy exists. To change the settings simply write the values described in this section to these files or display the files to check for the current setting.