In the latest Windows 8 blog post, on September 9, Gabe Aul, director of program management in Windows, authored "Delivering fast boot times in Windows 8" said that " We designed Windows 8 so that you shouldn't have to boot all that often (and we are always going to work on reducing the number of required restarts due to patching running code). But when you do boot we want it to be as fast as possible."
"Our solution is a new fast startup mode which is a hybrid of traditional cold boot and resuming from hibernate," revealed Aul.
Aul writes "The key difference for Windows 8: as in Windows 7, we close the user sessions, but instead of closing the kernel session, we hibernate it. Compared to a full hibernate, which includes a lot of memory pages in use by apps, session 0 hibernation data is much smaller, which takes substantially less time to write to disk.[…]Using this technique with boot gives us a significant advantage for boot times, since reading the hiberfile in and reinitializing drivers is much faster on most systems (30-70% faster on most systems we've tested)."
"It's faster because resuming the hibernated system session is comparatively less work than doing a full system initialization, but it's also faster because we added a new multi-phase resume capability, which is able to use all of the cores in a multi-core system in parallel, to split the work of reading from the hiberfile and decompressing the contents. For those of you who prefer hibernating, this also results in faster resumes from hibernate as well."
"It's probably worth mentioning quickly how we treat the hiberfile--if you read this and immediately went and did a dir /s /ah hiberfile.sys you would have found that it's a pretty big file on disk. The hiberfile is sized by default at 75% of physical RAM. The file is essentially a reservation for hibernation data that will be written out as the system is dropping into hibernation. Typically much less space is actually used, and in the case of our fast startup usage, it's typically ~10-15% of physical RAM but varies based on drivers, services, and other factors. The system also treats the hiberfile slightly differently than other files on disk, for example, the Volume Snapshot service ignores it (a small performance benefit.) You can disable hibernation and reclaim this space by running powercfg /hibernate off from an elevated command prompt. But be aware that if you do this, it will disable hibernation completely, including some nice capabilities like fast startup as well as hybrid sleep, which allows desktop systems to do both a sleep and hibernate simultaneously so if a power loss occurs you can still resume from the hibernated state. You can also run powercfg /hibernate /size and specify a value between 0 and 100 for the percentage of physical RAM to reserve for the hiberfile - but be careful! Specifying too small a size can cause hibernation to fail."
Another important thing to note about Windows 8's fast startup mode is that, while "we don't do a full "Plug & Play" enumeration of all drivers, we still do initialize drivers in this mode," Aul explained.
"Those of you who like to cold boot in order to "freshen up" drivers and devices will be glad to know that is still effective in this new mode, even if not an identical process to a cold boot."
"This new fast startup mode will yield benefits on almost all systems, whether they have a spinning HDD or a solid state drive (SSD), but for newer systems with fast SSDs it is downright amazing." Check out the video below," Aul said.
Aul also noted "One thing you'll notice in the video was how fast the POST handoff to Windows occurred. Systems that are built using Unified Extensible Firmware Interface (UEFI) are more likely to achieve very fast pre-boot times when compared to those with traditional BIOS. This isn't because UEFI is inherently faster, but because UEFI writers starting from scratch are more able to optimize their implementation rather than building upon a BIOS implementation that may be many years old. The good news is that most system and motherboard manufacturers have begun to implement UEFI, so these kinds of fast startup times will be more prevalent for new systems."