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I've heard many times over that OSX's system for handling files does away with the need to defragment my HDD unlike in Windows. However, I tried out Drive Genius 3 and it shows that 75% of my HDD that's less than a year old is fragmented. Apple genius bars supposedly use Drive Genius in their yearly Applecare tuneups..

Who should I believe?

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I'd like to know the answer to this as well. Apple's site says defragmentation is not necessary (support.apple.com/kb/HT1375) but I don't see how that's possible. A contiguous file will be read more quickly than a fragmented file in any situation, as far as I know. –  Huskehn Aug 11 '12 at 9:32
    
Apple and friends say that the filesystem apple uses (HFS+) is very good at not leaving fragmenting files, so they wouldn't need defragmenting in the first place. –  James Aug 11 '12 at 18:35
    
@James This post maybe helpful apple.stackexchange.com/questions/829/… –  Simon Apr 28 '13 at 15:18
    
@James Possible duplicate superuser.com/questions/17428/do-i-need-to-defragment-mac-os-x –  Simon Apr 28 '13 at 15:20
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1 Answer

When a file is opened on an HFS+ volume, the following conditions are tested:

  • The file is less than 20 MB in size
  • The file is not already busy
  • The file is not read only
  • The file is fragmented (the eighth extent descriptor in its extend record has a non-zero block count)
  • The system uptime is at least 3 minutes

If all the above are satisfied, the file is relocated (de-fragmented) on-the-fly.


Source: Bonus contents of "What is Mac OS X?", Amit Singh (© 1994-2010)


So you should act in order to prevent files from fragmenting too much (partitions/compression/etc) and, if needed, defragment with one of those third-party tools.

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protected by slhck Sep 1 '13 at 7:25

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