Great explanation here:
http://windowsitpro.com/articl.....fetch.html

The answer lies in the fact that during typical system boot or application startup, the order of faults is such that some pages are brought in from one part of a file, then from another part of the same file, then pages are read from a different file, then perhaps from a directory, and so on. This jumping around results in *moving the heads around on the disk*. Microsoft has learned through analysis that this slows boot and application startup times. By prefetching data from a file or directory all at once before accessing another one, this scattered seeking for data on the disk is greatly reduced or eliminated, thus improving the overall time for system and application startup.

For XP like performance on Vista, set prefetch to ON (3), if you find superfetch is being a bit too memory / HDD intensive, disable superfetch (0)

It is quite annoying to have the memory manager attempt to trim working sets or attempt to trim working set to the pagefile when Available Memory (really demand zero pages, free and zero page lists but that’s another story) exhaust and forces OS performance issues.

Superfetch tends to be great for pure business environment, but for a gaming rig/entertainment rig it gets wrong more than it gets right. I have had extensive conversations with the MS internal dev team responsible for the code, and they appear to understand (some are gamers themselves), but without getting Xperf traces where they can actually see Superfetch “goof” they don’t quite know what to tune. So most of them either modify EnableSuperfetch to help with this problem themselves as a workaround.

In case any were wondering “why dont’ they fix this!?” Oh, Windows 7 is just as bad as Vista despite the significant overhaul. It’s less stupid and makes fewer mistakes but still makes them – so be prepared to reset this value on Win7 just as I did when I upgraded from Vista SP2. FYI, YMMV, and the usual qualifiers.

In spite of the similar names, SuperFetch and Prefetch are really quite different features. The “EnablePrefetcher” setting is for Prefetch and has nothing to do with SuperFetch.

Prefetch was introduced with XP and was used to optimize both boot and application launch times. It does not preload anything and it is not a cache. The belief that it is a cache is very widespread but is totally wrong. The default setting for “EnablePrefetcher” is optimum and you will gain nothing by changing it. “Cleaning” the prefetch folder is not useful and will impair performance.

Superfetch was new in Vista and it does preload applications. It does this before the anticipated need, not at boot time. It has nothing to do with improving boot time. Some people find this feature useful, others do not. If it is not useful you may freely disable it. But do not attempt to control it’s operation with Prefetch parameters.

Larry Miller
Microsoft MCSA

http://msdn.microsoft.com/en-u......5%29.aspx

Granted, that refers to the EnablePrefetcher key rather than the EnableSuperfetch key, but there is no plausible reason for Microsoft to swap the values around for the latter.

Also, I tried setting EnableSuperfetch = 2 and discovered that the Superfetch service refrains from constructing the per-user databases that are normally found under %WINDIR%\Prefetch. Specifically, it does not create those that have a .db extension *and* reference the SID of the applicable user account:

AgGlUAD_P_%SID%.db
AgGlUAD_S_%SID%.db

This is entirely consistent with the intended outcome: for Superfetch not to cache application usage patterns (which is actually done on a per-account basis despite a number of erroneous remarks to the contrary that I’ve seen here and there). The results are easily reproducible as it is quite safe to delete the contents of the Prefetch directory – as long as the Superfetch service is stopped beforehand.

Today, I was transferring about 100 GB from my non-OS drive to another machine over my gige network.. MY machine went nuts. Within 5 mins, the machine stalled to the point where the mouse pointer didnt move in response to me moving the mouse. The file transfer was still going on, as I could monitor it from the other machine. About 10 mins after the file transfer completed, my computer SLOWLY started becoming responsive again. Curious as to what the hell happened, I opened up process monitor and tried the file transfer again, until my computer locked up. Turns out the pagefile is being read/written a considerable amount and my 4 gigs of memory being completely filled within 5 mins of the transfer. Stopped the file transfer, (computer still thrashing like crazy and stalling the computer).. stopped the superfetch process and just like that, the thrasing stopped completely and the computer came back alive. Tried again the file transfer and no more stalling.

I dunno what the hell superfetch was doing, but if it was trying to cache my 100 GB file transfer.. THAT’S RIDICULOUS…