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Managing the Power Used by Idle Disks in Servers Running the Solaris 8 OS or Above

January 2008

Each Serial-Attached SCSI (SAS) disk in your server can consume several watts while idle and spinning at full speed. If multiple disks (including external disks) are idle, the power cost can be significant.

The amount of power that can be saved varies according to the disk model. To spin down a SAS disk can save two to four watts. Spinning down older SCSI disks can save even more.

Disk spin-up is automatic in the Solaris Operating System, and it is triggered by a disk read-write request. The disadvantage in spinning down a disk is that it takes about 10 seconds for the disk to spin up and be ready to handle an I/O request.

Note: Please refer to your platform documentation to see if this feature is supported.

Guidelines for Disk Power Management

• Situations in which it is advisable to power-manage disk:

  • The disk is not used. (The disk is neither mounted nor accessed as a raw device.)
  • The disk is part of a group of disks reserved for performing scheduled backups.
  • A specific disk is busy during a predictable period and idle during the rest of the time.

• Situations in which it is not advisable to power-manage a disk:

  • The disk is a boot disk.
  • The disk is accessed by mission-critical tasks where time-to-response is important.
  • The disk is in service to global time-zone applications.

Enabling Solaris Disk Power Management

1. Become superuser.

2. Run /usr/sbin/pmconfig -r to unconfigure and reset. You need to apply the -r option only once per OS installation.

3. Edit /etc/power.conf. The following line explicitly enables the autopm feature in the Solaris OS:

autopm enable

Note: Only one uncommented line that starts with the token autopm is allowed.

4. Identify the disks to be powered down.

The following example uses the format(1M) command to show all present disks. Since disk 0 is the boot disk, disk 1 and 2 are selected to be subject to power saving.

# format
 Searching for disks...done

 AVAILABLE DISK SELECTIONS:
     0. c1t0d0 <SUN72G cyl 14087 alt 2 hd 24 sec 424>
        /pci@0/pci@0/pci@2/scsi@0/sd@0,0
     1. c1t1d0 <SUN72G cyl 14087 alt 2 hd 24 sec 424>
        /pci@0/pci@0/pci@2/scsi@0/sd@1,0
     2. c1t3d0 <SUN72G cyl 14087 alt 2 hd 24 sec 424>
        /pci@0/pci@0/pci@2/scsi@0/sd@3,0
....

5. Specify the idle threshold for selected disks in /etc/power.conf.

Disk idle threshold indicates the length of time that a disk has been inactive. The idle threshold can be set per disk, and is tunable. See the man page power.conf(4) for more information.

Continuing the example in Step 4, the following lines specify that if any of the selected disks has been inactive for 10 minutes, it will be spun down:

device-thresholds  /pci@0/pci@0/pci@2/scsi@0/sd@1,0    10m
device-thresholds  /pci@0/pci@0/pci@2/scsi@0/sd@3,0    10m

Note: The idle threshold in power.conf applies at all times. You cannot specify a power management window (such as hours of day or days of weeks) in power.conf. To work around this limitation, use a cron job. For further information about cron, see the man page cron(1M).

6. Disable power management for unselected devices by adding the following line in /etc/power.conf:

system-threshold        always-on

7. Run /usr/sbin/pmconfig to make the changes effective.

For More Information

Here are additional resources:

• The Sun training course SI-375, Writing Device Drivers for the Solaris Operating Environment, at http://www.sun.com/training/

• Support:

  • Register your Sun gear
  • Services
  • SunSolve Online

• Discussions, such as the Solaris OS forums

• Related documents at http://docs.sun.com:

  • pmconfig(1M) man page
  • power.conf(4) man page
  • Writing Device Drivers, which is part of the Solaris 10 Software Developer Collection
  • Using Power Management, which is part of the Solaris 7 User Collection and the Solaris 2.6 User Collection

• Events of interest to users of Sun products:

  • Worldwide Developer Events
  • Current Events

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