Windows 7 / Getting Started

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File System Fragmentation

As files are created, deleted, and modified over time, their size and physical location on the hard disk will change. If a file size needs to increase and the hard disk doesn't have room directly adjacent to the existing file, the file system automatically places the new portion of the file where it can find the room and then marks the necessary structures so that the file system can find the entire file when an application needs it. The file is now in two (or more) fragments.

Fragmentation is normal behavior and is completely transparent to both applications and users. The problem is that over time, more and more files become fragmented and even highly fragmented, increasing the amount of time that it takes for the hard disk controller to locate all of the fragments. Not only does this slow down file access, but it also places additional stress on the hard disk itself.

By default, Windows Vista and Windows 7 will defragment the hard drive at 1:00 A.M. every Wednesday. If the computer is off at this time, defragmentation will start shortly after the computer boots next. Ideally, defragmentation will run when the computer is not in use, minimizing the performance impact. However, the user impact is minimal because the defragmenter uses both low CPU priority and low-priority input/output (I/O).

Unlike earlier versions of Windows, Windows 7 recognizes solid-state drives (SSDs) and disables automatic defragmentation. Defragmentation does not improve the performance of SSDs and can decrease the lifetime of the SSD by unnecessarily reading and writing data.

Defragmentation Algorithm Improvements

Many systems administrators have been captivated by the graphical defragmentation displays in previous versions of Windows. You'll notice the graphics are gone in Windows Vista and Windows 7. Unfortunately, displaying the layout of files and highlighting files that had even one fragmentation made many performancefocused administrators obsessed with eliminating every single fragmented file.

Fragmentation does reduce disk performance, but having a few fragments in a large file doesn't make a difference-even years of reading and writing a large file with a single fragment would never add up to a significant amount of time. For this reason, Microsoft tweaked the defragmentation algorithm so that it does not defragment a file if a segment is longer than 64 MB. In those circumstances, the relatively significant effort required to rearrange files just to combine two 64-MB fragments isn't worth the effort, so Windows doesn't bother.

If you run a different defragmentation tool (including the defragmenter in Windows XP), those fragments will show up, and they'll probably look significant because the fragmented file is so large. (Typically, the entire file appears red if it has even a single fragment.) Trust the algorithm, though-a few fragments don't matter.

To defragment a file system or configure the automatic disk defragmentation schedule manually, follow these steps:

1. Click Start and then click Computer.
2. Right-click the drive and then click Properties.
3. Click the Tools tab and then click Defragment Now. The Disk Defragmenter appears.
4. In the Disk Defragmenter dialog box, click Defragment Disk to begin defragmentation.

You can continue to use the computer while defragmentation takes place, but it might be a little slower. You can also adjust the defragmentation schedule for a single computer from this interface.

For more complete control of defragmentation, you can use the command-line defragmentation tool, Defrag.exe, from an elevated command prompt. Defrag.exe has the following syntax, which has changed since Windows Vista.

Defrag <volume> | /C | /E <volumes> [/A | /X | /T] [/H] [/M] [/U] [/V]

The options for Defrag.exe are:

• <volume> The drive letter or mount point of the volume to defragment.
• /C Defragment all local volumes on the computer.
• /E Defragment all local volumes on the computer except those specified.
• /A Display a fragmentation analysis report for the specified volume without defragmenting it. Analysis reports resemble the following.
  

      Post Defragmentation Report:
  
  	Volume Information:
  		Volume size 		= 68.56 GB
  		Free space 		= 58.78 GB
  		Total fragmented space	= 0%
  		Largest free space extent = 31.64 GB
  
  	You do not need to defragment this volume
  

• /X Perform free-space consolidation. Free-space consolidation is useful if you need to shrink a volume, and it can reduce fragmentation of future files.
• /T Track an operation already in progress on the specified volume.
• /H Run the operation at normal priority instead of the default low priority. Specify this option if a computer is not otherwise in use.
• /M Defragment multiple volumes simultaneously, in parallel. This is primarily useful for computers that can access multiple disks simultaneously, such as those using SCSIor SATA-based disks rather than disks with an IDE interface.
• /U Print the progress of the operation on the screen.
• /V Verbose mode. Provides additional detail and statistics.