Btrfs: do not allocate chunks as agressively

Swinging this pendulum back the other way.  We've been allocating chunks up
to 2% of the disk no matter how much we actually have allocated.  So instead
fix this calculation to only allocate chunks if we have more than 80% of the
space available allocated.  Please test this as it will likely cause all
sorts of ENOSPC problems to pop up suddenly.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/extent-tree.c

@@ -3504,7 +3504,8 @@ static int should_alloc_chunk(struct btrfs_root *root,
 	 * and purposes it's used space.  Don't worry about locking the
 	 * global_rsv, it doesn't change except when the transaction commits.
 	 */
-	num_allocated += global_rsv->size;
+	if (sinfo->flags & BTRFS_BLOCK_GROUP_METADATA)
+		num_allocated += global_rsv->size;
 
 	/*
 	 * in limited mode, we want to have some free space up to
@@ -3518,15 +3519,8 @@ static int should_alloc_chunk(struct btrfs_root *root,
 		if (num_bytes - num_allocated < thresh)
 			return 1;
 	}
-	thresh = btrfs_super_total_bytes(root->fs_info->super_copy);
 
-	/* 256MB or 2% of the FS */
-	thresh = max_t(u64, 256 * 1024 * 1024, div_factor_fine(thresh, 2));
-	/* system chunks need a much small threshold */
-	if (sinfo->flags & BTRFS_BLOCK_GROUP_SYSTEM)
-		thresh = 32 * 1024 * 1024;
-
-	if (num_bytes > thresh && sinfo->bytes_used < div_factor(num_bytes, 8))
+	if (num_allocated + alloc_bytes < div_factor(num_bytes, 8))
 		return 0;
 	return 1;
 }