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I bought an SSD (SandForce 2), created a tiny 25GB partition on it for Windows and installed Windows 7 64-bit. In order to free disk space, I enabled compression on the drive using the Properties entry in the context menu for the drive in Explorer.

Prior to compressing I had around 5GB of free space. After compression I had 4GB, so compression was not working for me. I figured this might have happened because of the built-in data compression of the SSD.

I decompressed the files again - after decompression, it left me with 7GB of free space! Better yet, after restarting, I had 10GB. What is happening here?

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-1 The compression ratio can’t depend on the type of drive, since NTFS is hardware-agnostic. – kinokijuf Feb 4 '13 at 17:26
@kinokijuf modern SSDs compress and deduplicate blocks on disk. Thus, more compressible data will take up less actual space than less compressible data. Since these are block-level operations, NTFS never enters that picture. – bastibe Feb 5 '13 at 12:19
but the free space shown comes from NTFS. – kinokijuf Feb 5 '13 at 17:04
up vote 7 down vote accepted

Differences of 2 or 5 gigabytes are tiny. Changes in size of swap file or temporary files created by programs easily account for that much or more.

Is your pagefile of static size, or do you let OS to book it when needed? Do you use Photoshop or other program that uses lots of disk space for temporary files?

You can check the pagefile settings at "Control Panel" / "System" / "Advanced system settings" / "Performance" / "Settings" / "Advanced" / "Virtual memory" / "Change". If any of the drives lists "System managed", then Windows allocates and frees the pagefile as needed.

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at that time, compressing/decompressing files was the only thing I did. I did not change any settings about the pagefile. – bastibe Sep 5 '11 at 10:33
Yes, but if you have set the pagefile to variable size, Windows can resize it on fly. That's why I asked you to report how you have set it. – Zds Sep 5 '11 at 11:06
As I said, I did not change any settings regarding the pagefile size. It is set to whatever Windows sets it when you install it. – bastibe Sep 5 '11 at 12:16
Then you most likely are using variable size, ie. Windows changes the size when it feels like it. – Zds Sep 5 '11 at 12:21
@paperflyer: It is variable by default. – surfasb Sep 5 '11 at 16:48

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