Difference between pages "FAT" and "SuperFetch"

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'''FAT''', or File Allocation Table, is a [[File Systems|file system]] that is designed to keep track of allocation status of clusters on a [[hard drive]].  Developed in 1977 by [[Microsoft]] Corporation, FAT was originally intended to be a [[File Systems|file system]] for the Microsoft Disk BASIC interpreter. FAT was quickly incorporated into an early version of Tim Patterson's QDOS, which was a moniker for "Quick and Dirty Operating System". [[Microsoft]] later purchased the rights to QDOS and released it under Microsoft branding as PC-DOS and later, MS-DOS. 
+
{{Expand}}
  
== Specification ==
+
SuperFetch is a performance enhancement introduced in [[Microsoft]] [[Windows|Windows Vista]] to reduce the time necessary to launch applications. SuperFetch works with the memory manager service in Windows to analyze memory usage patterns over time to determine the optimal memory content for a given user for a date or time of day. This differs from the [[Prefetch]] technique used in Microsoft Windows XP, which preloads data into memory without analyzing usage patterns.
  
FAT is described by Microsoft in [[Media:Fatgen103.doc|Microsoft's FAT32 specification]]. Despite the name, the document includes descriptions of FAT12 and FAT16.
+
From [http://msdn.microsoft.com/en-us/library/windows/hardware/dn653317(v=vs.85).aspx]: SuperFetch prioritizes the following kinds of pages to remain in memory:
 +
* Pages of applications that are used most frequently overall.
 +
* Pages of applications that are commonly used when resuming:
 +
** After extensive hibernation (for example, first thing in the morning).
 +
** After shorter periods of sleep or hibernation (for example, after lunch).  
  
Closely related standards are: ECMA 107 and ISO/EIC 9293, which only cover FAT12 and FAT16, and also are somewhat more restricted than the file system described by Microsoft's document.
+
If SuperFetch detects that the system drive is a fast SSD (as measured by Windows Experience Index Disk score), then SuperFetch turns off [[ReadyBoot]], [[ReadyBoost]], and the SuperFetch service itself.
  
== Structure==
+
== Components ==
 +
=== Robust performance ===
 +
Component of SuperFetch called robustness, or robust performance to watch for specific file I/O access that might harm system performance by populating the standby lists with unneeded data.
  
{| style="text-align:center;" cellpadding="3" border="1px"
+
== Configuration ==
| Boot sector
+
| More reserved<br/> sectors (optional)
+
| FAT #1
+
| FAT #2
+
| Root directory<br /> (FAT12/16 only)
+
| Data region<br /> (rest of disk)
+
|}
+
  
=== Boot Record ===
+
Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the [[Registry]] value [http://www.codinghorror.com/blog/archives/000688.html]:
When a computer is powered on, a POST (power-on self test) is performed, and control is then transferred to the [[Master boot record]] ([[MBR]]).  The [[MBR]] is present no matter what file system is in use, and contains information about how the storage device is logically partitioned.  When using a FAT file system, the [[MBR]] hands off control of the computer to the Boot Record, which is the first sector on the partition. The Boot Record, which occupies a reserved area on the partition, contains executable code, in addition to information such as an OEM identifier, number of FATs, media descriptor (type of storage device), and information about the operating system to be booted. Once the Boot Record code executes, control is handed off to the operating system installed on that partition.
+
 
+
=== FATs ===
+
The primary task of the File Alocation Tables are to keep track of the allocation status of clusters, or logical groupings of sectors, on the disk drive.  There are four different possible FAT entries: allocated (along with the address of the next cluster associated with the file), unallocated, end of file, and bad sector.
+
 
+
In order to provide redundancy in case of data corruption, two FATs, FAT1 and FAT2, are stored in the file system. FAT2 is a typically a duplicate of FAT1. However, FAT mirroring can be disabled on a FAT32 drive, thus enabling any of the FATs to become the Primary FAT. This possibly leaves FAT1 empty, which can be deceiving.
+
 
+
=== Root Directory ===
+
The Root Directory, sometimes referred to as the Root Folder, contains an entry for each file and directory stored in the file system.  This information includes the file name, starting cluster number, and file size. This information is changed whenever a file is created or subsequently modified. Root directory has a fixed size of 512 entries on a hard disk and the size on a floppy disk depends.  With FAT32 it can be stored anywhere within the partition, although in previous versions it is always located immediately following the FAT region.
+
 
+
=== Data Area ===
+
 
+
The Boot Record, FATs, and Root Directory are collectively referred to as the System Area.  The remaining space on the logical drive is called the Data Area, which is where files are actually stored.  It should be noted that when a file is deleted by the operating system, the data stored in the Data Area remains intact until it is overwritten.
+
 
+
=== Clusters ===
+
In order for FAT to manage files with satisfactory efficiency, it groups sectors into larger blocks referred to as clusters. A cluster is the smallest unit of disk space that can be allocated to a file, which is why clusters are often called allocation units. Each cluster can be used by one and only one resident file. Only the "data area" is divided into clusters, the rest of the partition is simply sectors. Cluster size is determined by the size of the disk volume and every file must be allocated an even number of clusters. Cluster sizing has a significant impact on performance and disk utilization. Larger cluster sizes result in more wasted space because files are less likely to fill up an even number of clusters.
+
 
+
The size of one cluster is specified in the Boot Record and can range from a single sector (512 bytes) to 128 sectors (65536 bytes). The sectors in a cluster are continuous, therefore each cluster is a continuous block of space on the disk.  Note that only one file can be allocated to a cluster.  Therefore if a 1KB file is placed within a 32KB cluster there are 31KB of wasted space. The formula for determining clusters in a partition is (# of Sectors in Partition) - (# of Sectors per Fat * 2) - (# of Reserved Sectors) ) /  (# of Sectors per Cluster).
+
 
+
=== Wasted Sectors ===
+
 
+
'''Wasted Sectors''' (a.k.a. '''partition [[slack]]''') are a result of the number of data sectors not being evenly distributed by the cluster size. It's made up of unused bytes left at the end of a file. Also, if the partition as declared in the partition table is larger than what is claimed in the Boot Record the volume can be said to have wasted sectors. Small files on a hard drive are the reason for wasted space and the bigger the hard drive the more wasted space there is. 
+
 
+
=== FAT Entry Values ===
+
<br>
+
FAT12<br>
+
<br>
+
0x000          (Free Cluster)<br>   
+
0x001          (Reserved Cluster)<br>
+
0x002 - 0xFEF  (Used cluster; value points to next cluster)<br>
+
0xFF0 - 0xFF6  (Reserved values)<br>
+
0xFF7          (Bad cluster)<br>
+
0xFF8 - 0xFFF  (Last cluster in file)<br>
+
<br>
+
FAT16<br>
+
<br>
+
0x0000          (Free Cluster)<br>
+
0x0001          (Reserved Cluster)<br>
+
0x0002 - 0xFFEF  (Used cluster; value points to next cluster)<br>
+
0xFFF0 - 0xFFF6  (Reserved values)<br>
+
0xFFF7          (Bad cluster)<br>
+
0xFFF8 - 0xFFFF  (Last cluster in file)<br>
+
<br>
+
FAT32<br>
+
<br>
+
0x?0000000              (Free Cluster)<br>
+
0x?0000001              (Reserved Cluster)<br>
+
0x?0000002 - 0x?FFFFFEF  (Used cluster; value points to next cluster)<br>
+
0x?FFFFFF0 - 0x?FFFFFF6  (Reserved values)<br>
+
0x?FFFFFF7              (Bad cluster)<br>
+
0x?FFFFFF8 - 0x?FFFFFFF  (Last cluster in file)
+
 
+
Note: FAT32 uses only 28 of 32 possible bits, the upper 4 bits should be left alone. Typically these bits are zero, and are represented above by a question mark (?).
+
 
+
==Versions==
+
 
+
There are three variants of FAT in existence: FAT12, FAT16, and FAT32.
+
 
+
=== FAT12 ===
+
*  FAT12 is the oldest type of FAT that uses a 12 bit file allocation table entry. 
+
*  FAT12 can hold a max of 4,084 clusters (which is 2<sup>12</sup> clusters minus a few values that are reserved for values used in  the FAT). 
+
*  It is used for floppy disks and hard drive partitions that are smaller than 16 MB. 
+
*  All 1.44 MB 3.5" floppy disks are formatted using FAT12.
+
*  Cluster size that is used is between 0.5 KB to 4 KB.
+
 
+
=== FAT16 ===
+
*  It is called FAT16 because all entries are 16 bit.
+
*  FAT16 can hold a max of 65,524 addressable units
+
*  It is used for small and moderate sized hard disk volumes.
+
 
+
=== FAT32 ===
+
FAT32 is the enhanced version of the FAT system implemented beginning with Windows 95 OSR2, Windows 98, and Windows Me.
+
Features include:
+
*  Drives of up to 2 terabytes are supported ([[Windows]] 2000 only supports up to 32 gigabytes)
+
*  Since FAT32 uses smaller clusters (of 4 kilobytes each), it uses hard drive space more efficiently. This is a 10 to 15 percent improvement over FAT or FAT16.
+
*  The limitations of FAT or FAT 16 on the number of root folder entries have been eliminated. In FAT32, the root folder is an ordinary cluster chain, and can be located anywhere on the drive.
+
*  File allocation mirroring can be disabled in FAT32. This allows a different copy of the file allocation table then the default to be active.
+
 
+
==== Limitations with [[Windows]] 2000 & [[Windows]] XP ====
+
* Clusters cannot be 64KB or larger.
+
* Cannot decrease cluster size that will result in the the FAT being larger than 16 MB minus 64KB in size.
+
* Cannot contain fewer than 65,527 clusters.
+
* Maximum of 32KB per cluster.
+
* ''[[Windows]] XP'': The Windows XP installation program will not allow a user to format a drive of more than 32GB using the FAT32 file system. Using the installation program, the only way to format a disk greater than 32GB in size is to use NTFS. A disk larger than 32GB in size ''can'' be formatted with FAT32 for use with Windows XP if the system is booted from a Windows 98 or Windows ME startup disk, and formatted using the tool that will be on the disk.
+
 
+
=== exFAT (sometimes incorrectly called FAT64) ===
+
exFAT (also know as Extended File Allocation Table or exFAT) is Microsoft's latest version of FAT and works with Windows Embedded CE 6.0, Windows XP/Server 2003 (with a KB patch, Vista/Server 2008 SP 1 & Later, and Windows 7.
+
Features include:
+
*  Largest file size is 2<sup>64</sup> bytes (16 exabytes) vs. FAT32's maximum file size of 4GB.
+
*  Has transaction support using Transaction-Safe Extended FAT File System (TexFAT). (Not released yet in Desktop/Server OS)
+
*  Speeds up storage allocation processes by using free space bitmaps.
+
*  Support UTC timestamps (Vista/Server 2008 SP1 does not support UTC, UTC support came out with SP2)
+
*  Maximum Cluster size of 32MB (Fat32 is 32KB)
+
*  Sector sizes from 512 bytes to 4096 bytes in size
+
*  Maximum FAT supportable volume size of 128PB
+
*  Maximum Subdirectory size of 256MB which can support up to over 2 million files in a singlr subdirectory
+
*  Uses a Bitmap for cluster allocation
+
*  Supports File Permissions (Not released yet in Desktop/Server OS)
+
*  Has been selected as the exclusive file system of the SDXC memory card by the SD Association
+
 
+
Although Microsoft has published some information on exFAT, there are more technical specifications available from third parties. For example, here is a  [http://paradigmsolutions.files.wordpress.com/2009/12/exfat-excerpt-1-4.pdf detailed presentation on exFAT].
+
 
+
Another published technical paper that goes in the internals in great detail is in the SANS Reading Room at: [http://www.sans.org/reading_room/whitepapers/forensics/rss/reverse_engineering_the_microsoft_exfat_file_system_33274 Reverse Engineering the Microsoft exFAT File System]
+
 
+
=== Comparison of FAT Versions ===
+
 
+
See the table at http://en.wikipedia.org/wiki/File_Allocation_Table for more detailed information about the various versions of FAT.
+
 
+
== Uses ==
+
Due to its low cost, mobility, and non-volatile nature, flash memory has quickly become the choice medium for storing and transferring data in consumer electronic devices. The majority of flash memory storage is formatted using the FAT file system.  In addition, FAT is also frequently used in electronic devices with miniature hard drives.
+
 
+
Examples of devices in which FAT is utilized include:
+
 
+
* [[USB]] thumb drives
+
* [[Digital camera|Digital cameras]]
+
* Digital camcorders
+
* Portable audio and video players
+
* Multifunction [[printers]]
+
* Electronic photo frames
+
* Electronic musical instruments
+
* Standard televisions
+
* [[PDAs]]
+
 
+
==Data Recovery==
+
Recovering directory entries from FAT filesystems as part of [[recovering deleted data]] can be accomplished by looking for entries that begin with a sigma 0xe5. When a file or directory is deleted under a FAT filesystem, the first character of its name is changed to sigma. The remainder of the directory entry information remains intact.
+
 
+
The pointers are also changed to zero for each cluster used by the file.  Recovery tools look at the FAT to find the entry for the file.  The location of the starting cluster will still be in the directory file.  It is not deleted or modified.  The tool will go straight to that cluster and try to recover the file using the file size to determine the number of clusters to recover.  Some tools will go to the starting cluster and recover the next "X" number of clusters needed for the specific file size.  However, this tool is not ideal.  An ideal tool will locate "X" number of available clusters.  Since files are most often fragmented, this will be a more precise way to recover the file.
+
 
+
An issue arises when two files in the same row of clusters are deleted.  If the clusters are not in sequential order, the tool will automatically receive "X" number of clusters.  However, because the file was fragmented, it's most likely that all the clusters obtained will not all contain data for that file.  If these two deleted files are in the same row of clusters, it is highly unlikely the file can be recovered.
+
 
+
==File [[Slack]]==
+
File [[slack]] is data that starts from the end of the file written and continues to the end of the sectors designated to the file. There are two types of file [[slack]], RAM slack and Residual [[slack]]. RAM slack starts from the end of the file and goes to the end of that sector. Residual slack then starts at the next sector and goes to the end of the cluster allocated for the file.  File slack is a helpful tool when analyzing a hard drive because the old data that is not overwritten by the new file is still in tact. Go to http://www.pcguide.com/ref/hdd/file/partSizes-c.html for examples.
+
 
+
 
+
<table border="1" cellspacing="2" bordercolor="#000000" cellpadding="4" width="468" bordercolorlight="#C0C0C0">
+
  <tr>
+
    <td width="101" bgcolor="#808080"><font size="2"><b><center>Cluster</center></b></font></td>
+
    <td width="177" bgcolor="#808080"><font size="2"><b><center>Sample Slack Space,
+
    50% Cluster Slack Per File</center></b></font></td>
+
    <td width="178" bgcolor="#808080"><font size="2"><b><center>Sample Slack Space,
+
    67% Cluster Slack Per File</center></b></font></td>
+
  </tr>
+
  <tr>
+
    <td width="101" bgcolor="#C0C0C0"><font size="2"><b><center>2 kiB</center></b></font></td>
+
    <td width="177"><font size="2"><center>17 MB</center></font></td>
+
    <td width="178"><font size="2"><center>22 MB</center></font></td>
+
  </tr>
+
  <tr>
+
    <td width="101" bgcolor="#C0C0C0"><font size="2"><b><center>4 kiB</center></b></font></td>
+
    <td width="177"><font size="2"><center>33 MB</center></font></td>
+
    <td width="178"><font size="2"><center>44 MB</center></font></td>
+
  </tr>
+
  <tr>
+
    <td width="101" bgcolor="#C0C0C0"><font size="2"><b><center>8 kiB</center></b></font></td>
+
    <td width="177"><font size="2"><center>66 MB</center></font></td>
+
    <td width="178"><font size="2"><center>89 MB</center></font></td>
+
  </tr>
+
  <tr>
+
    <td width="101" bgcolor="#C0C0C0"><font size="2"><b><center>16 kiB</center></b></font></td>
+
    <td width="177"><font size="2"><center>133 MB</center></font></td>
+
    <td width="178"><font size="2"><center>177 MB</center></font></td>
+
  </tr>
+
  <tr>
+
    <td width="101" bgcolor="#C0C0C0"><font size="2"><b><center>32 kiB</center></b></font></td>
+
    <td width="177"><font size="2"><center>265 MB</center></font></td>
+
    <td width="178"><font size="2"><center>354 MB</center></font></td>
+
  </tr>
+
</table>
+
 
+
The diagram above demonstrates the larger the cluster size used, the more disk space is wasted due to slack. This suggests it is better to use smaller cluster sizes whenever possible.
+
 
+
==FAT Advantages==
+
*  Files available to multiple operating systems on the same computer
+
*  Easier to switch from FAT to [[NTFS]] than vice versa
+
*  Performs faster on smaller volumes (< 10GB)
+
*  Does not index files, which causes slightly higher performance
+
*  Performs better with small cache sizes (< 96MB)
+
*  More space-efficient on small volumes (< 4GB)
+
*  Performs better with slow disks (< 5400RPM)
+
 
+
==FAT Disadvantages==
+
*  FAT has a fixed maximum number of clusters per partition, which means as the hard disk gets bigger the size of each cluster must increase, creating more slack space
+
*  Doesn't natively support many abilities of [[NTFS]] such as on-the-fly compression, [[encryption]], or advanced security using access control lists
+
*  [[NTFS]] recommended by [[Microsoft]] for volumes larger than 32GB
+
*  FAT slows down as the number of files on the disk increases
+
*  FAT usually fragments files more
+
*  FAT does not allow for indexing of files for faster searching
+
*  FAT does not support user quotas
+
*  FAT has minimal security features including no access control list (ACL) capability.
+
 
+
== FAT date and time values ==
+
Python code to convert a FAT date and time value into a Python datetime object.
+
 
<pre>
 
<pre>
import datetime
+
Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
 +
Value: EnableSuperfetch
 +
</pre>
  
def FromFatDateTime(fat_date_time):
+
A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, <tt>services.msc</tt> [http://tiredblogger.wordpress.com/2007/03/27/superfetch-not-so-super-for-gaming/].
  """Converts a FAT date and time into a timestamp.
+
  
  FAT date time is mainly used in DOS/Windows file formats and NTFS.
+
== File Formats ==
  
  The FAT date and time is a 32-bit value containing two 16-bit values:
+
Data for SuperFetch is gathered by the <tt>%SystemRoot%\System32\Sysmain.dll</tt>, part of the Service Host process, <tt>%SystemRoot%\System32\Svchost.exe</tt>, and stored in a series of files in the <tt>%SystemRoot%\Prefetch</tt> directory [http://www.microsoft.com/technet/technetmag/issues/2007/03/VistaKernel/]. These files appear to start with the prefix <tt>Ag</tt> and have a <tt>.db</tt> extension. Note that there are likely more SuperFetch database files named differently, presumably all using the .db extension.
    * The date (lower 16-bit).
+
      * bits 0 - 4:  day of month, where 1 represents the first day
+
      * bits 5 - 8:  month of year, where 1 represent January
+
      * bits 9 - 15: year since 1980
+
    * The time of day (upper 16-bit).
+
      * bits 0 - 4: seconds (in 2 second intervals)
+
      * bits 5 - 10: minutes
+
      * bits 11 - 15: hours
+
  
  Args:
+
The format of the SuperFetch database files is not fully known, there is available unofficial partial specification [http://blog.rewolf.pl/blog/?p=214] and open source (GPL) dumper for .db files [http://code.google.com/p/rewolf-superfetch-dumper/]. For more information see [[Windows SuperFetch Format|SuperFetch Format]].
    fat_date_time: The 32-bit FAT date time.
+
  
  Returns:
+
The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed [http://channel9.msdn.com/showpost.aspx?postid=242429].
    A datetime object containing the date and time or None.
+
  """
+
  day_of_month = (fat_date_time & 0x1f) - 1
+
  month = ((fat_date_time >> 5) & 0x0f) - 1
+
  year = (fat_date_time >> 9) & 0x7f
+
 
+
  if day_of_month < 0 or day_of_month > 30 or month < 0 or month > 11:
+
    return None
+
 
+
  fat_date_time >>= 16
+
 
+
  seconds = (fat_date_time & 0x1f) * 2
+
  minutes = (fat_date_time >> 5) & 0x3f
+
  hours = (fat_date_time >> 11) & 0x1f
+
 
+
  if hours > 23 or minutes > 59 or seconds > 59:
+
    return None
+
 
+
  return datetime.datetime(
+
      1980 + year, month, day_of_month, hours, minutes, seconds)
+
</pre>
+
  
== External links ==
+
The SuperFetch service is managed by the File Information FS MiniFilter service. It appears that most of the SuperFetch database files are updated (written) when the service is shut down. AgAppLaunch.db is also written when the service starts.
* http://en.wikipedia.org/wiki/File_Allocation_Table
+
* http://www.microsoft.com
+
* http://www.ntfs.com
+
* http://www.ntfs.com/ntfs_vs_fat.htm
+
* http://support.microsoft.com/kb/q154997/#XSLTH3126121123120121120120
+
* http://www.dewassoc.com/kbase/hard_drives/boot_sector.htm
+
* http://home.teleport.com/~brainy/fat32.htm
+
* http://www2.tech.purdue.edu/cpt/courses/cpt499s/
+
* http://home.no.net/tkos/info/fat.html
+
* http://web.ukonline.co.uk/cook/fat32.htm
+
* http://www.ntfs.com/fat-systems.htm
+
* http://www.microsoft.com/whdc/system/platform/firmware/fatgen.mspx
+
* http://support.microsoft.com/kb/q140418
+
  
=== ExFAT ===
+
== See Also ==
* [http://en.wikipedia.org/wiki/ExFAT Wikipedia: ExFAT]
+
* [[Prefetch]]
* [http://www.active-undelete.com/xfat_volume.htm exFAT File System]
+
* [[ReadyBoost]]
* [http://www.sans.org/reading-room/whitepapers/forensics/reverse-engineering-microsoft-exfat-file-system-33274 Reverse Engineering the Microsoft exFAT File System], by [[Robert Shullich]], December 1, 2009
+
* [[ReadyBoot]]
* [http://paradigmsolutions.files.wordpress.com/2009/12/exfat-excerpt-1-4.pdf Extended FAT file system], by [[Jeff Hamm]], December 2009
+
* [[Windows SuperFetch Format|SuperFetch Format]]
* [http://www.slideshare.net/overcertified/demystifying-the-microsoft-extended-fat-file-system-exfat Demystifying the Microsoft Extended FAT File System (exFAT)], by [[Robert Shullich]], September 20, 2010
+
* [[Windows]]
* [http://aut.researchgateway.ac.nz/bitstream/handle/10292/4123/LeY.pdf Windows Phone 7 : Implications For Digital Forensic Investigators], by [[Yung Anh Le]], 2012
+
  
=== textFAT ===
+
== External Links ==
* [http://msdn.microsoft.com/en-us/library/ee490643(v=winembedded.60).aspx TexFAT Overview (Windows Embedded CE 6.0)], by [[Microsoft]]
+
* [http://technet.microsoft.com/en-us/magazine/2007.03.vistakernel.aspx Inside the Windows Vista Kernel: Part 2], by [[Mark Russinovich]], March 2007
* [http://www.ntfs.com/exfat-textFAT-padding.htm TexFAT Padding Directory Entry]
+
* [http://en.wikipedia.org/wiki/Windows_Vista_I/O_technologies#SuperFetch Wikipedia: Windows Vista I/O technologies - SuperFetch]
 +
* [http://channel9.msdn.com/showpost.aspx?postid=242429 Channel 9 Interview with Michael Fortin of Microsoft on SuperFetch]
 +
* [http://www.informationweek.com/news/showArticle.jhtml?articleID=196902178 Microsoft Predicts The Future With Vista's SuperFetch] from Information Week
 +
* [http://jessekornblum.com/presentations/dodcc08-2.pdf DC3 Presentation: My You Look SuperFetching], by Jesse Kornblum
  
 
== Tools ==
 
== Tools ==
=== exFAT ===
+
=== Open Source ===
* [http://code.google.com/p/exfat/ Open Source exFAT file system implementation]
+
* [https://code.google.com/p/rewolf-superfetch-dumper/ rewolf-superfetch-dumper]
  
[[Category:File Systems]]
+
[[Category:Windows]]

Revision as of 06:01, 28 June 2014

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Please help to improve this article by expanding it.
Further information might be found on the discussion page.

SuperFetch is a performance enhancement introduced in Microsoft Windows Vista to reduce the time necessary to launch applications. SuperFetch works with the memory manager service in Windows to analyze memory usage patterns over time to determine the optimal memory content for a given user for a date or time of day. This differs from the Prefetch technique used in Microsoft Windows XP, which preloads data into memory without analyzing usage patterns.

From [1]: SuperFetch prioritizes the following kinds of pages to remain in memory:

  • Pages of applications that are used most frequently overall.
  • Pages of applications that are commonly used when resuming:
    • After extensive hibernation (for example, first thing in the morning).
    • After shorter periods of sleep or hibernation (for example, after lunch).

If SuperFetch detects that the system drive is a fast SSD (as measured by Windows Experience Index Disk score), then SuperFetch turns off ReadyBoot, ReadyBoost, and the SuperFetch service itself.

Components

Robust performance

Component of SuperFetch called robustness, or robust performance to watch for specific file I/O access that might harm system performance by populating the standby lists with unneeded data.

Configuration

Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the Registry value [2]:

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
Value: EnableSuperfetch

A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, services.msc [3].

File Formats

Data for SuperFetch is gathered by the %SystemRoot%\System32\Sysmain.dll, part of the Service Host process, %SystemRoot%\System32\Svchost.exe, and stored in a series of files in the %SystemRoot%\Prefetch directory [4]. These files appear to start with the prefix Ag and have a .db extension. Note that there are likely more SuperFetch database files named differently, presumably all using the .db extension.

The format of the SuperFetch database files is not fully known, there is available unofficial partial specification [5] and open source (GPL) dumper for .db files [6]. For more information see SuperFetch Format.

The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed [7].

The SuperFetch service is managed by the File Information FS MiniFilter service. It appears that most of the SuperFetch database files are updated (written) when the service is shut down. AgAppLaunch.db is also written when the service starts.

See Also

External Links

Tools

Open Source