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The title pretty much explains the question.

Aside from the obvious (speed), is it generally safe or advisable to compress via NTFS compression the C:\Windows\Installer folder given that it's within the Windows folder?

I'm using an SSD so personally the speed issue isn't that important to me (plus I'm running Win7 within a VM so I'm accustomed to higher I/O latency).

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Are you referring to NTFS file compression or archiving these files (like a .zip)? – Oliver Salzburg Jul 26 '12 at 20:57
NTFS file compression. I'll update the question to reflect this. – Chad Levy Jul 26 '12 at 20:57
I did it and run into a bunch of problems, messing with hidden system folders is not advisable unless explicitly approved by Microsoft. – Vojtěch Dohnal Jun 26 at 9:01
up vote 16 down vote accepted

Except for speed issues (there is a lot of discussion here on that topic with some saying it is actually beneficial to performance in some ways) there is no issues in using NTFS compression a current Windows installation disk and all it's folders in it.

  • Your files are there, available, they do not really change to a secret compressed file or something, this is done at the file system level
  • Performance is not worst depending on opinions and some tests
  • They are recoverable using the Windows repair utilities or by connecting your disk to another computer (besides, nothing important is ever kept inside c:\Windows, your important files should be else where...)

Microsoft has put together some guide-lines about NTFS compression, have a look at them.

I have used NTFS compression for ages now on complete file systems on several clients and Windows versions and never had an issue because of the compressed filesystem.

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Yes, there is nothing special about the Installer directory that requires them to be uncompressed.

In the old days, there were piles of installer files like unwise.exe, unvise32.exe, uninstall.exe, etc. in the \Windows\ directory; sometimes shared, sometimes not. The \Windows\Installer folder is the Windows Installer cache, which allows each program to maintain its own installation program, similar to how the WinSxS folder helps to avoid DLL Hell.

In general, it is used to store/cache the basic installer for programs, so that when you want to modify an installed program, it runs from there and allows you to uninstall or possibly even do a repair without needing the original installation media, and so there should be no adverse affects from setting it to use NTFS compression.

I’ll leave the answer to whether you can compress/archive them as ZIP, RAR, etc. for others who find their way here, looking for that.

Some programs choose to store their entire installer which of course is undesirable, but fortunately those are rare; most just store a smaller installer file (like unwise32.exe in the old days) with the basic installation information. Most programs don’t use the files in the Installer folder at all and they only come into play when you make a change with Add/Remove Programs.

Occasionally however, you may find a program that actively uses some of the files, particularly by setting the icon of their shortcuts (in the Start menu, desktop, etc.) to point to the ARPPRODUCTICON.exe file in its Installer cache.

In most cases, you can compress or move the files and put them back when you want to make a change (modify, repair, uninstall). In fact, the folder is ultimately a cache of installation files, so you could even delete them and just use the original installation media.

NB: it is not only programs that cache their installation files in the Installer folder; Windows itself keeps patches and service-pack data in there as well, so if you delete them, updates may not work, and theoretically, you might have to re-install Windows. Don’t just move/delete the entire contents willy-nilly; take a look at what is in there.

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Referring to your last sentence, are not the compressed files (using NTFS file compression) still accessible while compressed? – Chad Levy Jul 26 '12 at 21:05
Oh, you mean compressed as in the compressed bit of NTFS? Yes, definitely. There’s nothing wrong with that at all. I thought you meant compressign them as in archiving it as a ZIP, RAR, etc. – Synetech Jul 26 '12 at 21:06
Yeah, sorry about that. I updated the question after your answer. – Chad Levy Jul 26 '12 at 21:09

SQL Server (and other databases too) may not be running normally if you install them on a compressed partition. They may simply fail to install.

Here is a post describing the scenario. I also encountered this on SQL Server 2014 Express.

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Could you add a source for that information? – Burgi May 11 at 9:28
The scenario is described here by another user: link. I also encountered this on SQL Server 2014 Express. – Ryan May 13 at 13:54
it was a hint for you to edit your answer to help you improve the quality of it ;) (also include a quote or two from your source) – Burgi May 13 at 14:59
Thanks! I'm quite new to Super User and I will commit to improving quality of my answer. – Ryan May 14 at 4:03

I'd advise against it, like Startup recovery might depend on those files, then you'd be unable to uncompress those files without another computer

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Do you have any sources for this? I am unable to find any references to starup recovery/repair relying on content in the Installer folder. Given that I alternate my Win7 install between bare-metal boots and VM boots (via Mac with BootCamp and VMWare Fusion), this certainly caught my attention since I'm more likely to encounter startup issues. – Chad Levy Jul 26 '12 at 21:14
NTFS compression is done at the volume level. Startup recovery will do fine with or without it. . . – surfasb Jul 27 '12 at 1:38
Etherway, the installer folder is a pretty important place . If Startup recovery has to reinstall anything, without the installer folder your stuck . Find another way to save space.. – Keithsoulasa Jul 27 '12 at 5:27

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