Why isn't hibernation enabled by default? Is there some downside to it?
Also, how reliably does it bring back everything how you left it?
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Well, to understand why sleep is preferred instead of hibernation, it's important to know what each of them is (source):
Sleep is commonly known as Standby in Windows system or S3 in ACPI. In Sleep mode, the power supply to non-essential and non-critical component is withheld, and most system operation is shutdown and stopped. All data in physical memory (RAM module) is still kept in internal memory, and whole system is placed in stand-by mode, which can be woke up and used almost immediately. In Sleep mode, the power load reduce considerably, saving a lot of energy. However, the power must not be cut off, and must be continue to supply to the computer. Once out of power, the system will have to start again just like a newly boot computer just started from power off state.
Hibernate, or S4 in ACPI, meanwhile will save the data in physical memory to hard disk drive (HDD), and then power off the computer. In Hibernate mode, a file named hiberfil.sys which has the same file size as the amount of system memory will be created on the local disk. When user wants to use the computer again, the computer will boot up and load back the state at the last hibernation. The advantage of Hibernation mode is that no power is wasted for maximum saving of power. In Hibernation dormancy, no electricity is consumed by system. Beside, restore from Hibernate is generally faster than computer reboot, and is totally different from fresh start, as users can return to the exact state of last hibernation with all programs running and documents opened intact, instead of empty desktop. The disadvantage of Hibernate is that after a period of time, there may have fragmentation of file. Users will need to defragment the volume that stores the hibernation file frequently.
Hibernation is slower than sleep because the whole contents of the RAM must be saved on disk when entering this state. Having a 4 GB file on your C: partition is not exactly the best thing, at times. That file must then be loaded back into RAM when exiting hibernation. Sleep is faster, nothing to save, nothing to load (mostly).
Reliability should be no concern for either sleep, or hibernation.
Hibernation uses disk space (obviously) and doesn't work on some (mostly older) hardware. But when it works, it's solid. You'll know after the first few hibernates whether it works fine on your hardware. There really is no risk to trying it. Many many people use it daily without problems, and only boot all the way down when Windows requires it, or if sloppy apps cause a crash.