xfs: increase XFS_BTREE_MAXLEVELS to fit the rmapbt

Source kernel commit: df3954ff72590fd20b68261a0c939e40fa3579ea

By my calculations, a 1,073,741,824 block AG with a 1k block size
can attain a maximum height of 9.  Assuming a record size of 24
bytes, a key/ptr size of 44 bytes, and half-full btree nodes, we'd
need 53,687,092 blocks for the records and ~6 million blocks for the
keys.  That requires a btree of height 9 based on the following
derivation:

Block size = 1024b
sblock CRC header = 56b
== 1024-56 = 968 bytes for tree data

rmapbt record = 24b
== 40 records per leaf block

rmapbt ptr/key = 44b
== 22 ptr/keys per block

Worst case, each block is half full, so 20 records and 11 ptrs per block.

1073741824 rmap records / 20 records per block
== 53687092 leaf blocks

53687092 leaves / 11 ptrs per block
== 4880645 level 1 blocks
== 443695 level 2 blocks
== 40336 level 3 blocks
== 3667 level 4 blocks
== 334 level 5 blocks
== 31 level 6 blocks
== 3 level 7 blocks
== 1 level 8 block

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/libxfs/xfs_btree.h b/libxfs/xfs_btree.h
index 8fc4ea08be1deef8c1ba28e4f1889fabd5ea8516..d1e87b00c3edb3b27bfa1b07c18a617b78bcb15e 100644 (file)
--- a/libxfs/xfs_btree.h
+++ b/libxfs/xfs_btree.h
@@ -133,7 +133,7 @@ do {    \
        }       \
 } while (0)
 
-#define        XFS_BTREE_MAXLEVELS     8       /* max of all btrees */
+#define        XFS_BTREE_MAXLEVELS     9       /* max of all btrees */
 
 struct xfs_btree_ops {
        /* size of the key and record structures */