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Subject[patch] multipage pagecache writeout

These patches:

implement multipage writeout from the pagecache. These patches require
the allocate-on-flush patches. The dalloc-30-ratcache patch is not a
requirement for the mpio series. But is recommended for
balls-to-the-wall how-fast-can-it-go testing.

Pages from the pagecache are given a disk mapping, are assembled into
large BIOs (up to half a megabyte) and these BIOs are injected direct
into the request layer.

These pages never have attached buffer_heads. The buffer layer is
completely bypassed for all write(2) data. As is, to some extent, the
request merging layer.

This patch should bypass the lru_list_lock contention problem, and the
ZONE_NORMAL-full-of-buffer_heads bug. (Well, this may require
multipage reads, too).

Future work includes:

- Implement buffer_head-less block_truncate_page().

- multipage reads.

A bit of a no-brainer, but first the current readahead code needs a
big shakeout.

Two additional patches are available:

- The get_request starvation fix for 2.5.

This patch also increases the request queue by a
lot. Which implies that we can have as much as 512 megabytes
of I/O underway per device. This may sound excessive, but
the locked- and dirty-page accounting in the delalloc patch
only permits this to happen if the machine is large enough to
cope with it.
- Pull the backing block for a new ext2 inode into
the buffercache when the inode is created. This fixes a
significant throughput problem with many-file writeout, where
the writer is continually interrupted by having to perform
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