Fragments of truth

Last month, Jon Honeyball mentioned the standalone disk defragmenter Diskeeper 9 in his Advanced Windows column, where he was very critical of some of the wording and ‘scare tactics’ used by that product. Specifically, he objected to its claiming that his system’s reliability could be degraded to a critical extent because of fragmentation of the hard disk.

Fragments of truth

I’m equally perturbed at the wording used in Diskeeper, as the statement on its Reliability tab says:


The computer’s reliability is severely affected on volume D:

The reliability level is at “Critical” for the following reason:

The MFT usage is currently 96 percent of the total MFT size, which indicates it is likely the MFT will become fragmented.

What worries me about this statement is that it is so easy to misunderstand what is being said. There is no problem with that figure of 96 per cent (which may differ from system to system, of course) – indeed, I would take no issue if it said 100 per cent – but problems really can arise if the Master File Table (MFT) becomes seriously fragmented. The MFT is a file that contains an entry for each of the other files stored on your hard disk. Since it is important that the MTF does not become fragmented, one-eighth of your hard disk space is allocated as ‘Reserved System Space’ for it to grow into. As files are added to the system, so the size of the MFT grows. However, this does not pose a problem for a while, because it can simply expand into its own contiguous disk area.

Let’s look at an example: say the MFT currently holds records for 100 files. At this point, it is full and so its usage is 100 per cent. You now delete four files, but the MFT does not shrink to reflect this; it simply empties the contents of four slots, which are now available to hold new files, and usage drops to 96 per cent. You now add five more files. The MFT fills its four empty slots, taking it back to 100 per cent usage and then grows to accommodate a further slot for the fifth file. Usage remains at 100 per cent and, because there is still a fair bit of space reserved for the MFT, that’s all contiguous.

Problems occur when the hard disk becomes so full there is no longer any contiguous free space to grow the MFT, at which point it becomes fragmented. A small amount of fragmentation is not an issue, but eventually you could end up with a hard disk so fragmented, both in terms of the data files and the MFT, that it takes for ever to open files and becomes a pain to work on. Generally speaking, it is not a good idea to fill a hard disk to capacity, assuming it is subject to normal daily traffic of newly created and deleted files. You always need to keep space free for temporary files, and you must leave enough space for a disk defragmenter to work.

If all your files are really small, you can get away with having less free space than if your system is populated with very large files. Even so, do not drop below 15-20 per cent free space, and 25 per cent would be better in some cases, depending on what the system’s being used for. Although one-eighth of the hard disk is set aside for the MFT, that does not mean one-eighth of space is no longer useable for your data. It still appears as free space when you check disk capacity and you do not feel its impact until it is all gone.

If you have a huge number of files under 8KB in size, you are likely to fill the Reserved System Space with the MFT file, and at this point MFT fragmentation will start to occur. Conversely, if you have a lot of large files, chances are you will run out of free disk space before it will have too much effect on the contiguity of the MFT. You will, however, find that your large files will now be occupying the area previously set aside as Reserved System Space (although NTFS has been designed to ensure this happens from the end of the reserved space, thus ensuring that the MFT file remains contiguous for as long as possible).

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