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Measuring Fragmentation in HFS+

Posted by pudge on from the bring-out-the-big-guns dept.

keyblob8K writes "Amit Singh takes a look at fragmentation in HFS+. The author provides numbers from his experiments on several HFS+ disks, and more interestingly he also provides the program he developed for this purpose. From his own limited testing, Apple's filesystem seems pretty solid in the fragmentation avoidance department. I gave hfsdebug a whirl on my 8-month-old iMac and the disk seems to be in good shape. I don't have much idea about ext2/3 or reiser, but I know that my NTFS disks are way more fragmented than this after similar amount of use."

4 of 417 comments (clear)

  1. HFS+ defrag source by revscat · · Score: 5, Informative

    As mentioned in the article, HFS+ does defragging on the fly when files are opened if they are less than 20MB. The source code for this is available here, as is a discussion about it that contains input from some Darwin developers.

    1. Re:HFS+ defrag source by Daniel_Staal · · Score: 5, Informative

      I believe the actual sequence is this:

      1. Get request for file
      2. Open File
      3. Buffer file to memory
      4. Answer request for file
      5. If needed, defragment file

      In other words, it defrangments after the file has been returned to the program needing it, as a background process. The buffer to memory is a pre-existing optimization, so the only real trade off is the background processor usage goes up. If you aren't doing major work at the time, you'll never notice. (And if you are doing major work, you probably are using files larger than 20MB in size anyway.)

      Files larger than 20MB just aren't defragmented, unless you have another tool to do it.

      --
      'Sensible' is a curse word.
    2. Re:HFS+ defrag source by shamino0 · · Score: 5, Informative
      HFS+ was one of the major features of the OS 8.1 update. OS 8.0 and earlier can't "see" HFS+ volumes- they see a tiny disk with a simpletext file titled "where have all my files gone?" which, if I remember correctly, gives a brief explanation that the disk is HFS+ and requires 8.1 or higher to view. :)

      And the person who came up with this idea was a genius. This is far far better than what most other operating systems do (refuse to mount the volume.)

      If I boot MS-DOS on a machine that has FAT-32 or NTFS volumes, I simply don't find any volume. I can't tell the difference between an unsupported file system and an unformatted partition. If the file system would create a FAT-compatible read-only stub (like HFS+ does), it would be much better for the user. Instead of thinking you have a corrupt drive, you'd know that there is a file system that your OS can't read.

  2. Re:NTFS is not so bad by MemoryDragon · · Score: 5, Informative

    Ntfs does not fragment that strongly as long as you dont hit the 90% full mark of your disk, once you reach that, see the files becoming fragmented in no time. NTFS uses the open space for write access and then probably relocates the files in time, once it hits 90% the open space usage algorithm does not seem to work anymore.