NTFS compressed folders: is it possible to tweak compression ratio?
Short answer
No, it's not possible at this time.
Long answer
Files and folders1 are compressed and decompressed by passing a FSCTL_SET_COMPRESSION control code and a compression state to the DeviceIoControl API function. The compression state can be one of the following:
COMPRESSION_FORMAT_NONE = 0
COMPRESSION_FORMAT_DEFAULT = 1
COMPRESSION_FORMAT_LZNT1 = 2
Any nonzero value means the target item is going to be compressed. From the official documentation:
The LZNT1 compression algorithm is the only compression algorithm implemented. As a result, the LZNT1 compression algorithm is used as the
DEFAULTcompression method.Source: FSCTL_SET_COMPRESSION control code
The LZNT1 algorithm is designed for speed, and there's no way to set a custom compression level.
1 Folders aren't actually compressed: their compression attribute only gives a default compression state to new files and subfolders.
Additional information
The NTFS file system volumes support file compression on an individual file basis. The file compression algorithm used by the NTFS file system is Lempel-Ziv compression. This is a lossless compression algorithm, which means that no data is lost when compressing and decompressing the file, as opposed to lossy compression algorithms such as JPEG, where some data is lost each time data compression and decompression occur.
On the NTFS file system, compression is performed transparently. This means it can be used without requiring changes to existing applications.
If you compress a file that is larger than 30 gigabytes, the compression may not succeed.
Source: File Compression and Decompression
The compression algorithms in NTFS are designed to support cluster sizes of up to 4 KB. When the cluster size is greater than 4 KB on an NTFS volume, none of the NTFS compression functions are available.
Source: File and Folder Compression
Further reading
- File Compression and Decompression
- File and Folder Compression
- Understanding NTFS Compression
- LZNT1 Algorithm Details
Generally it's not possible, as indicated in and31415's answer.
However Microsoft has added some new NTFS compression options and algorithms in Windows 10, so now there's a way to change the compression ratio:
COMPACT [/C | /U] [/S[:dir]] [/A] [/I] [/F] [/Q] [/EXE[:algorithm]]
[/CompactOs[:option] [/WinDir:dir]] [filename [...]]
...
/EXE Use compression optimized for executable files which are read
frequently and not modified. Supported algorithms are:
XPRESS4K (fastest) (default)
XPRESS8K
XPRESS16K
LZX (most compact)
The new algorithms are intended for the new Compact OS feature (as you can see in the /CompactOs option above). The idea is to compress read-only, backup and less frequently accessed system files with the highest ratio. The recovery partition is removed and the compressed file will then be used for both executing (if it's already the latest version) and recovery purpose
The LZX algorithm is quite efficient at archiving. And despite the option name and description are all about "executable files", the option can be applied to any files
E:\test>compact /a
Listing E:\test\
New files added to this directory will not be compressed.
1050909 : 1050909 = 1.0 to 1 sometext.txt
...
E:\test>compact /c /exe:lzx sometext.txt
Compressing files in E:\test\
sometext.txt 1050909 : 176128 = 6.0 to 1 [OK]
1 files within 1 directories were compressed.
1,050,909 total bytes of data are stored in 176,128 bytes.
The compression ratio is 6.0 to 1.
However unlike the old algorithm they don't support on-the-fly editing, so writing back to the file uncompresses it.
E:\test>echo x >> sometext.txt
E:\test>compact /a
Listing E:\test\
New files added to this directory will not be compressed.
1050913 : 1050913 = 1.0 to 1 sometext.txt
An important note is that you can't set the folder to mark new files to be compressed automatically using those new algorithms, because as stated in the help part of compact (emphasis mine)
/C Compresses the specified files. Directories will be marked
so that files added afterward will be compressed ***unless /EXE
is specified***.
/U Uncompresses the specified files. Directories will be marked
so that files added afterward will not be compressed. If
/EXE is specified, only files compressed as executables will
be uncompressed; if this is omitted, only NTFS compressed
files will be uncompressed.
Therefore if you want to use them for back-up files you might have to run a script to compress manually after backing up or periodically after some time
Unfortunately this new in Windows 10, therefore it can't be used in older versions. However NTFS-3g does support it, therefore you won't have problem accessing it from Linux. If you don't use Windows 10 you can run a Linux live USB or Windows 10 PE to open those files