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Old 28. Jul 2013, 12:53 PM   #4 (permalink)
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Join Date: Jul 2010
Location: New Zealand
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Originally Posted by Nymphus_A View Post
I read that about 20% of your drive should be left free. Is that true OR a myth.
Originally Posted by J_L View Post
20% probably means free space in your system partition to keep Windows running smoothly, it can work with less.
Ha, I only answered the post because of this question and promptly forgot to answer it.

@Nymphus_A. As J_L says it is the partition or logical drive that is being talked about. It is not worth leaving part of your physical drive unused.

The rule of thumb is a blunt instrument that is designed to protect the average user. In the 1980s, 20% was the rule of thumb for DOS/Windows FAT file systems primarily because fragmentation of files degraded performance so much. Nowadays it is not so much of a concern and most recommendations would be somewhere between 10 and 15%.

Why has it changed? One reason is that, in general, the size of the average file has increased much more slowly than the size of the average drive. About twenty years ago a song was 5MB and disks were 1GB so each song took up 0.5% of space. Nowadays a song is 5MB and disks are 1TB so each song file uses 0.0005% of storage. The management of the disk changes dramatically because impact of one extra file is much less now. Of course, nowadays, if you have large HD video/movie files then you might only fit 100 on a 1TB drive and the impact of changes is greater.
Another reason is that newer file systems are also more robust.

The % of disk space you leave free will be higher for:
  • the proportion of files being updated (created or modified). At one extreme, I used to have a drive for my music with nothing else on it. I filled it to 99% and higher because I hardly ever added to it. The disk was usually on read from and not written to. If I left 20% of the disk space free then I would need have a disk or partition 25% larger.
  • the largest size of files being updated. Often you need at least enough space to temporarily create a transitional copy of the file. For example, many programs including Microsoft Office programs work on temporary files and not on the original document.
  • the creation of temporary files and working space. If Windows creates virtual memory files that are fragmented then system performance will be reduced.
  • the requirements to install software. For example, a program I wanted to install wouldn't because it wanted 15GB free space when I only had 5GB free.
  • how well the file system copes with fragmentation. The old FAT system was particularly affected by this and it is still used on many USB sticks and flash memory devices.
  • the consequences if the disk fills and operations are unable to be completed. Ie. what happens if I can't save my work, Windows can't create a temporary file, etc.
Better to light a candle ... than to curse the darkness.
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