Device and Bus Power Management FAQ

What kind of power savings can I expect with ALPM?
It depends on your system, and where you measure power. On our tests, measured at the wall socket with a power meter, we've seen anywhere from .5 to 1.5 Watts of power savings. A whitepaper showing power savings measured at the system level is available here:
http://www.intel.com/technology/itj/2005/volume09issue01/vol09_iss01.pdf

Does ALPM work on for non-AHCI compliant controllers?
No.

If I configure my SATA drive for legacy or non-AHCI mode, I consume less power. Why?
Native AHCI provides a high transfer rate, as well as new functionality. This high-speed interface comes at the cost of additional power demand.

Does ALPM replace disk power management features?
No - ALPM is link power management, which is independent of disk power management.

My disk I/O seems very slow when ALPM is configured. Why?
The link must transition out of the link power management states in order to return to the "active" state. These transition can have a latency of up to 10 microseconds for the "Partial" state, and 10 milliseconds for the "Slumber" state.

What is the difference in power usage between "Partial" and "Slumber" power states?
In our tests, Partial saves about half as much power as Slumber, but with significantly less impact on performance.

When would I use Partial vs. Slumber states?
Use Slumber if you think the link will be idle for a significant amount of time, use Partial if you think the link will continue to be in use, but you are willing to sacrifice some performance for power savings. It is recommended to use Partial for the best Power vs. Performance tradeoff.

Is ALPM available in the upstream Linux kernel yet?
Not yet, although patches for testing are available here: http://www.kernel.org/pub/linux/kernel/people/kristen/patches/SATA/alpm and documentation of how to enable ALPM will be available in the kernel source Documentation tree when it is integrated.

How does Asynchronous Notification save power?
User space applications would ordinarily need to poll to see if new media has been inserted. This keeps your CPU busy and out of lower power states. If the kernel can tell user space when new media has been inserted, polling is not required.

Is AN available in the upstream kernel?
Not yet, but if you would like to see the work in progress, you can find patches at:
http://www.kernel.org/pub/linux/kernel/people/kristen/patches/SATA/AN

Documentation on this feature will be available in the kernel source tree Documentation subdirectory when it is integrated.

Is AN supported on non-AHCI mode controllers?
Theoretically, yes. In Linux, the support is only being implemented for the AHCI driver.