Walking the btree can trigger a number of single block synchronous reads.
This patch does btree readahead during operations that are likely to be long
and sequential.  So far, that only includes directory reads and truncates, but
it can make both much faster.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>