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Windows Vista SP2, (uncompressed 100 GB Drive C is 50% full and Windows file indexing is on):

Drive C, where all the program files, \Windows folder and all my data is located is half full, enjoys fast disk access. I also have disk caching optimization turned on. If I were to compress all the files on drive C, would disk access be faster?.. At what point (% full) should I consider compressing drive C?.. Should I only compress the data or all the files on this drive?

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Somewhat related, but that question does not address at which point of % full should disk compression be performed, nor does it address which files should or should not be compressed. – Frank R. Aug 2 '12 at 23:53
Read the post which @Breakthough mentioned again. Performance gain or loss is minimal and depends a lot on your setup. (E.g. a fast CPU and a slow disc might even increase IO performance). As to how full: No influence, or, if you wish, when you feel you have to little space left. – Hennes Aug 3 '12 at 0:08
@Hennes: Which post? – Tom Wijsman Aug 9 '12 at 18:09
I am pretty sure there was another comment on this article. Either that or I had to many open tabs. Is there a way to see older versions of this page? – Hennes Aug 9 '12 at 19:58

You are assuming for there to be a relation between how full your disk is and compressing at such point, in essence it is not going to matter how full your disk is as compressing will have the same effect when you have barely any files or a ton of files. What does have an effect though is data being further from the center of your disk as well as fragmentation, which both come as you get a fuller disk (and only on a HDD it might matter). Both are, however, not problems you can't fix.

So, this in essence turns your question in "does compressing slow down disk performance?"

For this question, we already have some Q&A but they are in essence subjective:

If you really want to know, measuring the performance difference will tell you whether it helps.

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1 for measure performance. Given the number of variables all we can make a educated guesses and suggestions. The true way to get an answer is to make a backup (ghost, acronos, FOG, CloenZilla, ...). Time the things you usually do. Compress and time again. – Hennes Aug 9 '12 at 20:00

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