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SubjectRe: NTFS-link streams?

(repost -- earlier post delivered to ether)

Christopher Vickery wrote:

> I'm interested in implementing a system that associates
> meta-data with inodes, and would like to know if it has
> already been done or is in the works.

None has. There's two different ways of doing it currently;
the BeOS way and the NT way. As you said, NT makes a
namespace augmentation, using the ":" character to
deliniate attribute names from file names. This is called
"named streams". BeOS does not do that, but provides
special accessor functions instead; this is called
"extended attributes." They both accomplish the same
goal though: keeping extra data about a file with the file.


Urban Widmark wrote:

> Erm. What is the point?
> If you read x.bmp do you eventually start reading x.bmp:thumbnail?

The point is to be able to store and manage extra data about a file
with the file. On BeOS and NT, if you copy, move, rename, etc. a
file, the metadata (stream/attribute data) stays with it. The Mac
uses its streams to store file type information, icon, icon
position, comments and more. That's all done in a standard way and
is transparent to applications simply wanting to read the data fork.

I'm not sure what your question is asking. You don't have to read
x.bmp:thumbnail if you do not want to. But if you want to view the
thumbnail, it's there. For instance, on the Mac, you can have a
comment stored with each file as an extended attribute. Applications
can choose to use this data if they want, but htye do not have to. It
simply makes things nicer for the user.


Alexander Viro wrote:
> There is a sane and completely portable scheme - use directories. It's
> less brittle and will work with the standard tool on normal filesystems.
> Why not use it?

Because it's more brittle. There is no reliable way to keep the extra
data associated with a file actually with the file. Using directories at
best keeps it _near_ the file. Plus, the implementation of "extended
attributes" using regular directories would probably be highly
application-dependant.


Alexander Viro wrote:
> The main problem is that rename(), unlink(), etc. are too
> fine-grained for such beasts. It's not that filesutils will barf on names.
> Just suppose that you are saying mv * foo/bar. Shell will substitute
> the whole whorehouse - filename1:attribute1, filename1:attribute2, etc.
> and in the best case mv will call rename() on each of them (just how should
> we interpret "move the attribute A of file B into directory foo/bar"?).
> In the worst it will try to _copy_ them and then remove the old ones.
> Moreover, you are going to confuse the hell of VFS and every program out
> there (let alone POSIX, etc.) if rename() on one name will move a bunch of
> other. It just does not happen.

The whole whorehouse wouldn't be moved, or shell-subsitutited. If you
had a file, "myfile", which had three streams named "1", "2" and "3", on a
streams- aware filesystem, all you see in "ls" is "myfile", not
"myfile:1", "myfile:2", etc. And to copy or move the file, you just "copy
myfile" and the streams go with it.

Linux can actually be equipped with both BeOS-type nad NT-like systems
for accessing named streams/extended attributes. A filesystem can tell the
VFS taht it supports named streams. Otherwise, named streams are not
supported. This is similar to NTFS vs FAT on Windows NT. If you copy a
file with streams from and NTFS volume to a FAT volume, the streams are
thrown away. There is no attempt made to support streams on a filesystem
that does not support them.

Similarly, an enhanced Linux VFS can support or not support streams,
depending on the capabilities of the underlying filesystem. The Macintosh
actually makes and attempt to preserve extended attributes on non-HFS
filesystems through "AppleDouble" and "AppleSingle" schemes. While that is
interesting, I prefer the cleaner NT way of dealing with it -- don't
kludge extra features onto that don't support them.

Granted, there will still be issues with userspace tools like cp. Rm and
mv will not have an issue, unless mv is moving across volumes. Cp and mv
will have to enumerate and copy all streams. Rm will not need to enumerate
streams; if you delete a file with streams, all the streams go with
it. The best move there would be to put functions in libc for copying
files, and have programs use it.

On filesystems that support fast indexing -- JFS, XFS, Reiser -- streams
can be used as in BeOS to provide indexing capabilities. BeOS email
applications, for instance, use extended attributes to store information
about a message -- the subject, sender, etc. This allows any application
to look up email messages by those criteria simply by reading the
attributes, rather than parsing mail headers.

-Michael



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