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SubjectNTFS primer - was: Re: NTFS-like streams?
Sorry for bringing this thread back into life but after having spent the 
past 5 or so hours reading its contents I feel it is necessary, since many
people don't seem to understand/know about NTFS and hence a lot of wrong /
pointless posts were made. - Obviously my holiday was badly timed... )-:

Here is a very basic summary of NTFS concepts which will, I hope, explain
why most approaches that have been outlined to sort out the streams/EAs
problem in Linux cannot work. - A very important point is the fact that a
stream is not an EA and neither of those is a user defined attribute. -
However NTFS supports all of them!

NTFS concepts+clear ups
-----------------------

0. The NTFS file system is a relational database of file records. (These
are stored in the $MFT file record. - Yes, the file system is really
recursive! Even the boot sector is a file and has a file record in the
$MFT, admittedly with a fixed on disk location...)

1. A file record (aka file) is a structured list of attributes (such as:
NTFS filename, MS DOS filename, Posix filename, standard file information,
security descriptor, named data attribute [aka streams], HPFS extended
attributes [aka EAs], user defined attributes [aka what some people call
extended attributes], quota information, symbolic link information, mount
point information, directory index information, directory index entry,
unique object Id, etc. - Note, this is somewhat simplified and many of
those attributes are actually one and the same/multiple copies of the same
but with different contents.)[Note, that the data parts of each of those
attributes is not necessarily stored in the file record itself.]

2. A directory is a file (and hence can have all possible attributes a
normal file can have - a nameless data attribute is not permitted however).
The filename of the root directory is "\". A directory file record contains
a list of index entries pointing to file records in the $MFT.

3. The streams people have been referring to are attributes of type $Data
with a specific name (e.g. file_name:data_attribute_name). The "normal"
file-contents people usually talk about are represented by a nameless $Data
attribute (name length = 0). These streams are used for implementing the
HFS file system on top of NTFS [done by the Macintosh services for NT].

4. The EAs most people have been referring to are attributes of type $EA
and are used mainly for HPFS compatibility.

5. User defined attributes are extensions to the file system and require a
file system filter to be added to the running kernel to deal with them. -
These are intended as non-MS extensions to NTFS.

6. The reparse point attribute is used in Win2k (NTFS v3.0) for symbolic
links and mount points.

7. Name space for filenames: there are three of them. MSDOS compatible
short names, Win32/NT compatible long names and Posix filenames. While
MSDOS and Win32/NT filenames coexist happily (and in fact MSDOS names are
auto generated for each Win32/NT long filename), a file with a Posix
filename will be invisible to all of the Windows NT tools which do not
specifically use the Posix subsystem. No auto generation of other names is
performed here.

8. Hard links are Posix compliant and are implemented by having several
filename attributes in a file record (one for each hard link) and by having
the same file record being referenced by different directory index entries
(again, one for each hard link). - Thus when deleting files what happens is
that you delete the directory index record + filename attribute and if no
more filename attributes are present you then proceed to actually deleting
the file.

Conclusions
-----------
You don't want to start emulating EAs using streams or vice versa since
they are distinct entities.

While the colon approach would work for streams as it does in WinNT it
doesn't cover EAs at all and in fact it can't do that. - A attr_{get,set}
API or an attr_ops structure (or something else?) will be required for EAs.
- I certainly can't see any way you can fuse the two together but I would
be happy to be shown the light. (-;

The only problem with the colon approach I can see is that Linux/UNIX
actually allow colon as a valid character in a file name. This will at the
very least cause massive confusion among end users. - Imagine Joe Doe with
a ext2 partition having named his/her mp3 files in the form: albumname:
songname.mp3. Joe then copies the files to an NTFS partition and does "ls"
and sees only one file called albumname with zero size (NTFS parsed the
colon to mean file albumname and named data attribute songname.mp3 for each
songname.mp3). At this point Joe panics and dies of a heart attack... )-:

And yes, the file size displayed in the command prompt when using the dir
command in NT only counts up the size of the unnamed data attribute...

That's all for now!

<please flame off list>

Regards,

Anton
--

"Education is what remains after one has forgotten everything he
learned in school." - Albert Einstein

--
Anton Altaparmakov Voice: 01223-333541(lab) / 07712-632205(mobile)
Christ's College eMail: AntonA@bigfoot.com
Cambridge CB2 3BU ICQ: 8561279
United Kingdom WWW: http://www-stu.christs.cam.ac.uk/~aia21/

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