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Hibernate is a feature of many computer operating systems where the contents of RAM are written to non-volatile storage such as a hard disk, as a file or on a separate partition, before powering off the computer. When the computer is restarted it reloads the content of memory and is restored to the state it was in when hibernation was invoked. Hibernating and later restarting is usually faster than closing down, later starting up, and starting all the programs that were running. Going into hibernation requires no user interaction; shutting down when programs are open either requires programs to be closed properly, or user interaction to force them to close, with possible loss of data.
To enable hibernation the hard disk must have sufficient free space to store all non-replaceable contents of RAM.
In some cases entering into hibernation can cause incorrect operation on restarting, due to problems with the hibernation software, or with devices or software which is not fully compliant. Hibernate causes connections to other devices to terminate.
To determine the true "savings" associated with hibernation as a network policy, organizations should weigh lost productivity associated with reestablishing the pre-hibernation state against the power savings created by the hibernation policy.
A hibernating machine uses no more power than one which is switched off—modern machines, even if switched off, often consume a little power allowing them to be woken on an alarm timer, by Wake-on-LAN, etc. Many systems also support a low-power "standby" mode in which the processing functions of the machine are powered down, using a little power to preserve the contents of RAM and support waking up; wakeup is almost instantaneous. If the contents of RAM are also copied to non-volatile storage we have the best of both worlds: if a key is pressed the machine wakes up instantaneously; if the machine is powered down it is effectively in a state of hibernation.
Hibernation is often used on laptops and can usually be set to happen automatically on a low battery alarm. Early implementations of hibernation used the BIOS, but modern operating systems usually handle hibernation. Hibernation is defined as sleeping mode S4 in the ACPI specification.[1]
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Operating system support
Microsoft Windows
Windows 95 supported hibernation through hardware manufacturer-supplied drivers and only if compatible hardware and BIOS were present. Since Windows 95 supported only Advanced Power Management (APM), hibernation was then known as Suspend-to-Disk. Windows 98 and later support ACPI. However, hibernation often caused problems since most hardware was not fully ACPI 1.0 compliant or did not have WDM drivers. There were also issues with the FAT32 file system.[2] Windows Me supports OS controlled hibernation and requires half the amount of disk space of the computer's RAM[citation needed].
Microsoft Windows 2000 and later support hibernation at the operating system level (OS-controlled ACPI S4 sleep state) without special drivers from the hardware manufacturer. A file, "hiberfil.sys" is used to store the contents of RAM when the computer hibernates, and is always the same size as the total RAM. A hidden system file resides in the root of the system partition, usually "C:\hiberfil.sys". This file is a full snapshot of the physical RAM memory compressed with an LZXPRESS algorithm, and contains further information including processor state. Although this file is undocumented, a security researcher, Matthieu Suiche, documented it[3] during Black Hat Briefings 2008 and has also provided a computer forensics framework to manage and convert this file into a readable memory dump.[4]
Windows XP further improved support for the hibernate function.[5] On Windows computers, hibernation is available only if all hardware is ACPI and Plug-and-play compliant, and additionally all drivers are PnP-compatible. The Microsoft Help and Support website also describes a command line instruction to invoke hibernation, in article #555569[6]; such command line instructions can be added to many keyboard instruction sets to hibernate computers at the touch of a (programmable) button.
Windows Vista's Fast Sleep and Resume feature saves the contents of volatile memory to hard disk before entering sleep mode. If power to memory is lost, it will use the hard disk to wake up. The user has the option of hibernating directly if they wish.
Hibernate is often under-used in business environments as it is difficult to enable post-deployment without resorting to third party software.[7] This omission by Microsoft has been criticized as having led to a huge waste in energy.[8]
Mac OS
On Macs, a feature known as Safe Sleep saves the contents of volatile memory to the system hard disk each time the Mac enters Sleep mode. The Mac can instantaneously wake from sleep mode if power to the RAM has not been lost. However, if the power supply was interrupted, such as when removing batteries without an AC power connection, the Mac would wake from Safe Sleep instead, restoring memory contents from the hard drive.[9] Because Safe Sleep's hibernation process occurs during regular Sleep, the Apple menu does not have a "hibernate" option.
Safe Sleep capability is found only in new Mac models starting with the October 2005 revision of the PowerBook G4 (Double-Layer SD). Mac OS X v10.4 or higher is also required.
Shortly after Apple started supporting Safe Sleep, Mac enthusiasts released a hack to enable this feature for much older Mac computers running Mac OS X v10.4.[10] Classical Mac OS once also supported hibernate, but this feature was dropped by Apple.[11]
Linux
In the Linux kernel, Hibernate or suspend-to-disk, suspend-to-ram, suspend-to-file is implemented by swsusp which is built into the 2.6 series. An alternative implementation is TuxOnIce which is available as patches for the 2.4 and 2.6 kernels. Other alternative implementations also exist, namely, uswsusp (user-space suspend). TuxOnIce provides advantages such as support for SMP, 4GB high mem and preemption. Currently, work is being done on merging TuxOnIce into the mainline kernel.[12]
See also
References
- ^ ACPI Specification Rev 3.0b
- ^ BIOS and Hibernate Issues for FAT32 File System
- ^ BlackHat USA 2008 - Windows hibernation file for fun and profit
- ^ SandMan Computer Forensics Framework
- ^ Windows Power Management: Instant PC availability and energy savings
- ^ Microsoft Help and Support - How To Put the System into hibernation or Standby from Run menu
- ^ Enable hibernate using group policy
- ^ EcoGeek - How Windows XP Wasted $25 Billion of Energy
- ^ Apple Support: Progress bar appears after waking from sleep
- ^ http://web.archive.org/web/20080105000720/http://andrewescobar.com/archive/2005/11/11/how-to-safe-sleep-your-mac/
- ^ http://docs.info.apple.com/article.html?artnum=59016
- ^ http://lists.tuxonice.net/lurker/message/20080716.071015.bd394e94.en.html
External links
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