btrfs: index buffer_tree using node size

So far we've been deriving the buffer tree index using the sector size.
But each extent buffer covers multiple sectors. This makes the buffer
tree rather sparse.

For example the typical and quite common configuration uses sector size
of 4KiB and node size of 16KiB. In this case it means the buffer tree is
using up to the maximum of 25% of it's slots. Or in other words at least
75% of the tree slots are wasted as never used.

We can score significant memory savings on the required tree nodes by
indexing the tree using the node size instead. As a result far less
slots are wasted and the tree can now use up to all 100% of it's slots
this way.

Note: This works even with unaligned tree blocks as we can still get
      unique index by doing eb->start >> nodesize_shift.

Getting some stats from running fio write test, there is a bit of
variance.  The values presented in the table below are medians from 5
test runs.  The numbers are:

  - # of allocated ebs in the tree
  - # of leaf tree nodes
  - highest index in the tree (radix tree width)):

ebs / leaves / Index |   bare for-next    |      with fix
---------------------+--------------------+-------------------
	post mount   |   16 /  11 / 10e5c |   16 /  10 / 4240
	post test    | 5810 / 891 / 11cfc | 4420 / 252 / 473a
	post rm	     |  574 / 300 / 10ef0 |  540 / 163 / 46e9

In this case (10GiB filesystem) the height of the tree is still 3 levels
but the 4x width reduction is clearly visible as expected. But since the
tree is more dense we can see the 54-72% reduction of leaf nodes. That's
very close to ideal with this test. It means the tree is getting really
dense with this kind of workload.

Also, the fio results show no performance change.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 6bc7f7ac381c..44e7ae4a2e0b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3397,6 +3397,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	fs_info->delalloc_batch = sectorsize * 512 * (1 + ilog2(nr_cpu_ids));
 
 	fs_info->nodesize = nodesize;
+	fs_info->nodesize_bits = ilog2(nodesize);
 	fs_info->sectorsize = sectorsize;
 	fs_info->sectorsize_bits = ilog2(sectorsize);
 	fs_info->csums_per_leaf = BTRFS_MAX_ITEM_SIZE(fs_info) / fs_info->csum_size;