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    SubjectRe: abstract file (multi-part one file or multiple files?)
    Date
    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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