Disk Power Management

SATA supports three power management states (SLUMBER, PARTIAL and MAX) and these are tunable via linux /sys file system. The specification states when drives are at PARTIAL and SLUMBER states, it need to switch to MAX state in about 10 us to 10 ms. If you add SW overhead the actual time to switch drive from SLUMBER to MAX is much higher. But power savings are considerable and its worth power managing the drives.

Linux 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. ALPM automatically sets the SATA link back to an active power state once I/O requests are queued to that link.

For example, user can set "/sys/class/scsi_host/host*/link_power_management_policy" to min_power, medium_power & max_performance. Each of above states correspond to SLUMBER, PARTIAL and MAX power states.

SATA PHY are based of CMOS digital logic and it uses almost no power in static condition. However the logic consumes power if gates switch logic leading to Higher Speed = More state transitions = Higher Power. Following state diagram describes SATA power transitions states in detail.