A. Windows uses the hiberfil.sys file for system hibernation. By design, the OS's built-in disk defragmenter utility and most third-party products, such as Executive Software's Diskeeper, won't defragment hiberfil.sys when you attempt optimize the disk that contains the file. However, SysInternals' free PageDefrag tool does defragment the file. If you don't want to use a third-party product to defragment hiberfil.sys, you can delete the hibernation file, defragment the disk, then recreate the hibernation file by performing the following steps:

  1. Start the Control Panel Power Options applet.
  2. Select the Hibernate tab.
  3. Clear the "Enable hibernation" check box, then click OK to delete the hiberfil.sys file.
  4. Start your defragmentation program to defragment the disk.
  5. After defragmentation is complete, start the Control Panel Power Options applet.
  6. Select the Hibernate tab.
  7. Select the "Enable hibernation" check box, then click OK to create the hiberfil.sys file.

Other files that the Windows disk defragmenter utility doesn't defragment are

  • Bootsect.dos
  • Safeboot.fs
  • Safeboot.csv
  • Safeboot.rsv
  • Memory.dmp
  • Files in the Recycle Bin (so it's a good idea to empty the Recycle Bin before you defragment a disk)
  • Pagefile.sys (third-party products can defragment this file)
  • Files that are in use when you run the disk defragmenter utility (so it's a good idea to shut down all running programs and any unneeded services before you run the utility)
  • If the disk that you're defragmenting has less than 15 percent free space available, the disk defragmenter won't completely defragment the volume; instead, the utility will complete only a partial defragmentation. Also, you can't defragment a drive that's marked as possibly having errors (run chkdsk <drive>: /f to resolve this problem).