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SubjectRe: abstract file (multi-part one file or multiple files?)
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Clayton Weaver writes:

> This is to some extent a disagreement over terminology. When we say
> "file", are we talking about the user interface or the implementation?

A file has these properties:

1. it can not be the current working directory of a process
2. it can be atomically unlinked
3. it has a single ACL (or UNIX permission bits, or whatever)
4. it can be hard-linked if the filesystem supports hard links
5. it has exactly one inode number
6. it has a visible name (ignoring hard links and deleted files)
7. it is of type S_IFREG (but note: directories may have streams too)
8. it has at least one stream, just as a process has threads

A stream has these properties:

1. it is a blob of data w/o OS-interpreted structure
2. it shared (inheritance WITH UPDATE) properties with others
3. it can not be independently hard linked
4. it has no normally visible name
5. st_mode has no meaning, except to describe the parent type
6. streams themselves do not have streams

> The complex document file is not a directory in the way that users
> understand directories, as an hierarchy of folders for storing objects
> that can be single-part or multi-part, but it needs to be implemented like
> a directory. The VFS must recognize that a multi-part storage object that
> is not explicitly a directory at user level has a directory implementation
> but has constraints on what can be nested in it that an explicit directory
> does not have.

One may implement complex document files using multi-streamed
files or, less efficiently, using single-streamed files. Using
multi-streamed files is a performance hack that may require some
sort of format-specific loadable module. Microsoft actually does
have such kernel support for the *.doc format. The concept isn't
at all alien to Linux; look at all the IP masquerading helpers.

(the kernel module exists only to solve the "lost stream" problem)
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