From c9dc4c6578502c2085705347375b82089aad18d0 Mon Sep 17 00:00:00 2001
From: Chris Mason <clm@fb.com>
Date: Sat, 4 Apr 2015 17:14:42 -0700
Subject: [PATCH] Btrfs: two stage dirty block group writeout

Block group cache writeout is currently waiting on the pages for each
block group cache before moving on to writing the next one.  This commit
switches things around to send down all the caches and then wait on them
in batches.

The end result is much faster, since we're keeping the disk pipeline
full.

Signed-off-by: Chris Mason <clm@fb.com>
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