Btrfs: tune btrfs unplug functions for a small number of devices

When btrfs unplugs, it tries to find the correct device to unplug
via search through the extent_map tree.  This avoids unplugging
a device that doesn't need it, but is a waste of time for filesystems
with a small number of devices.

This patch checks the total number of devices before doing the
search.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 8efc123d222b..1bb54d69fbb2 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1201,6 +1201,16 @@ void btrfs_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
                 return;
 
         inode = mapping->host;
+
+        /*
+         * don't do the expensive searching for a small number of
+         * devices
+         */
+        if (BTRFS_I(inode)->root->fs_info->fs_devices->open_devices <= 2) {
+                __unplug_io_fn(bdi, page);
+                return;
+        }
+
         offset = page_offset(page);
 
         em_tree = &BTRFS_I(inode)->extent_tree;