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Having just read Scott Hanselman's latest blog entry, Guide to Freeing up Disk Space under Windows 7, he suggests turning on NTFS compression which I already do for a number of less travelled folders that contain static files such as downloads or images.

However I am wondering if it's wise to turn on NTFS compression for the whole of my %USERPROFILE%\AppData folder?

My system drive is a 128 GB SSD residing in a Dell Precision T5400 3Ghz Quad Core Xeon workstation so I ought not to notice the extra cycles used to compress and decompress files on their way to and from the disk.

However would there be any good reasons not to do this?

In fact could I safely compress the whole of my %USERPROFILE% folder?

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As a developer I would say no. This is because applications (and even web-sites like Google) can use their application folders extensivly and as time goes on Microsoft are encouraging developers to go away from the registry as a information store (using application folders (SpecialFolders) instead. This will mean these folders are used more and more. With the overhead of NTFS compression and un-compression (in my oppinion) to compress these directories would not be wise. Microsoft say:

"Because NTFS compression is processor-intensive, the performance cost is more noticeable on servers, which are frequently processor-bound. Heavily loaded servers with a lot of write traffic are poor candidates for data compression. However, you may not experience significant performance degradation with read-only, read-mostly, or lightly loaded servers.

If you run a program that uses transaction logging and that constantly writes to a database or log, configure the program to store its files on a volume that is not compressed. If a program modifies data through mapped sections in a compressed file, the program can produce "dirty" pages faster than the mapped writer can write them. Programs such as Microsoft Message Queuing (also known as MSMQ) do not work with NTFS compression because of this issue.

Because user home folders and roaming profiles use lots of read and write operations, Microsoft recommends that you put user home folders and roaming profiles on a volume that does not have NTFS compression on the parent folder or on the volume root. Individual users may still enable compression on their folders, but the overall number of compressed files and folders is smaller. On servers that host compressed volumes, you should use careful performance monitoring to determine whether the CPU has enough capacity to support the compress/decompress operations that are being performed."

I hope this helps.

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