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3.8. Aggressive Link Power Management
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:
- This mode sets the link to its lowest power state (SLUMBER) when there is no I/O on the disk. This mode is useful for times when an extended period of idle time is expected.
- This mode sets the link to the second lowest power state (PARTIAL) when there is no I/O on the disk. This mode is designed to allow transitions in link power states (for example during times of intermittent heavy I/O and idle I/O) with as small impact on performance as possible.
medium_powermode 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.
- ALPM is disabled; the link does not enter any low-power state when there is no I/O on the disk.To check whether your SATA host adapters actually support ALPM you can check if the file
/sys/class/scsi_host/host*/link_power_management_policyexists. 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.