[I'm re-sending this since it didn't reach the mailing list due to the
SourceForge outage.]
There is not too much information specifically about this feature available yet.
You can try googling "Windows 10" "System compression" to find some articles.
If you are looking for information about the data format, it is not yet
documented in the context of the system compression feature but it seems that
Microsoft lifted the format of the compressed data directly from the Windows
Imaging (WIM) file format.
One way to create such files for testing is to use the Windows 10 version of the
"compact" program. It has a new option for compressing files using one of the
new formats:
/exe:xpress4k
/exe:xpress8k
/exe:xpress16k
/exe:lzx
The format is designed for write-once, read-many files, such as executable
files. If you try to write to such a file on Windows, Windows immediately
decompresses it and turns it into a standard uncompressed file. There is no
need for manual cluster allocation as the feature is not implemented directly in
NTFS.
However, for reading, the compressed files can be accessed randomly with "chunk"
granuality. Each chunk can be decompressed independently. If, say, you want to
read starting from byte offset 1000000 and the chunks are 8192 bytes, then you
know you need to read starting from chunk (1000000/8192) = 122. Then you can
load the offsets of chunks 122, and any later chunks that may be needed, from
the "chunk table" at the beginning of the file. Those will tell you where in
the file the chunks are and what their compressed sizes are.
Eric
On Thu, Jul 16, 2015 at 09:59:46AM +0200, Jean-Pierre André wrote:
> Hi Eric,
>
> Interesting.
>
> Where can I find more information about this feature,
> and how can I create such files on Windows 10 ?
>
> Glancing at your code, I do not see anything related
> to (sparse) cluster allocation. Does that mean these
> files are not seekable and must be read/written
> sequentially ?
>
> Regards
>
> Jean-Pierre
>
> Eric Biggers wrote:
> >Hello,
> >
> >I've made an experimental fork of ntfs-3g that supports reading the "System
> >Compressed" files that are / will be supported by Windows 10. This feature
> >allows rarely-modified files to be stored using XPRESS or LZX compression,
> >with
> >stronger compression than the LZNT1 compression built into NTFS. Windows 10
> >will supposedly enable it on selected files automatically.
> >
> >Microsoft designed this feature to use a reparse point which redirects
> >access to
> >a named data stream, which avoided changing NTFS itself. The format of the
> >compressed stream is identical to that of a compressed resource stored in a
> >Windows Imaging (WIM) archive.
> >
> >I suspect it will be a while before NTFS-3g support would be useful to more
> >people and it ultimately may not be worthwhile adding it at all (especially
> >since this is a reparse-point based feature and therefore is not part of NTFS
> >itself, and it takes quite a bit of code to support), but I thought I'd post
> >this in case anyone else is interested.
> >
> >The source code is available as the "system_compression" branch of
> >https://github.com/ebiggers/ntfs-3g.git.
> >
> >Eric
>
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