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    SubjectRe: fs specific union in struct buffer head? Optimal implementation of allocate on flush seems to require a b_parent field in struct buffer_head.
    I want to first implement allocate, then implement re-allocate.  Note the synergy
    between this, and online shrinking the fs, and journaling, and wandering logs,
    and implementing online reordering for higher performance during the night.


    "Stephen C. Tweedie" wrote:

    > Hi,
    > On Fri, 15 Oct 1999 23:22:23 +0400, Hans Reiser <>
    > said:
    > > Vladimir and I are discussing how to implement allocate on flush in our next
    > > major version (not for 2.4) of reiserfs.
    > >> If you are going to reallocate on flush unformatted nodes you must be able
    > >> to:
    > Remember, there is a huge difference between allocate-on-flush and
    > reallocate-on-flush. In the former case, you simply don't have
    > buffer_heads present until the final flush: everything has to be
    > maintained in the filesystem itself or the VM, not in the buffer
    > cache, until the final flush (because the buffer cache is really only
    > for storing blocks whose on-disk location is known).
    > In the reallocate case, a buffer_head b_fs_private pointer to
    > fs-specific metadata would allow the fs to deal with this
    > independently of the buffer cache layer itself. For
    > allocate-on-flush, though, you need to be able to look up the
    > information from the VM, not from the buffer cache.
    > --Stephen

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