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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.

Hans

"Stephen C. Tweedie" wrote:

> Hi,
>
> On Fri, 15 Oct 1999 23:22:23 +0400, Hans Reiser <reiser@idiom.com>
> 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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