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    SubjectRe: Question on the VFS inode structure.
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    Alexander Viro writes:
    > On Wed, 27 Oct 1999, T V Govind wrote:
    > > I'm new to linux and am currently in the process of trying to
    > > understand the kernel source (file system code specifically). i have the
    > > following question, the VFS inode structure has a union (shown below)
    > > that is a union of the "private data" of all those file systems that are
    > > currently supported on linux. Is there any specific reason why this
    > > union is maintained within the inode structure itself. i mean the
    > > structure is kind of not-specific to any particular file system and is
    > > also defined as part of the VFS layer on linux.
    >
    > The stuff will be switched to external allocation as soon as the pagecache
    > changes will settle down.

    I don't remember exactly what you proposed for this, but if you are
    intending to replace the inode structure with a generic one which has
    a pointer to a separately allocated per-fs structure then here's a
    warning. Digital UNIX used to that but had to change it exactly the
    opposite way for performance reasons. Its vnode structure used to
    have a v_data pointer to a separately allocated per-fs "private"
    structure. However, performance was bad because of the pointer
    following and the extra fragmentation. They ended up reuniting the two
    so that they could allocate the whole (generic + private) structure in
    one chunk and then made v_data point to directly after the generic
    part (i.e. the start of the no-longer-separate private part).

    They did the same with their ungodly mixture of BSD and Mach
    structures for tasks and threads: they had separate struct task,
    struct proc and struct utask which they combined into a single struct
    super_task (though still with pointers between the supposedly
    "separate" bits) and they had separate struct thread and struct
    np_uthread which they combined into struct super_thread.

    --Malcolm

    --
    Malcolm Beattie <mbeattie@sable.ox.ac.uk>
    Unix Systems Programmer
    Oxford University Computing Services

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