Re: [PATCH 20/47] xfs: increase XFS_BTREE_MAXLEVELS to fit the rmapbt

Darrick J. Wong
2016-08-02T20:06:51Z

On Tue, Aug 02, 2016 at 02:48:46PM -0400, Brian Foster wrote:
> On Wed, Jul 20, 2016 at 09:58:17PM -0700, Darrick J. Wong wrote:
> > 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.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
>
> I believe you.. :) but could you add a bit more detail to the commit log
> just to describe the calculations (e.g., how many records per leaf
> block, keys per node block) so 1.) I don't have to try and figure it out
> ;) (and probably get it wrong) and 2.) so we have a reference in the git
> repo going forward?

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

Yay latency....

(Hey, at least this isn't ext4 which has no lower bound on block fill...)

--D

>
> Brian
>
> > fs/xfs/libxfs/xfs_btree.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >
> > diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
> > index b8bbb21..629ef4f 100644
> > --- a/fs/xfs/libxfs/xfs_btree.h
> > +++ b/fs/xfs/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 */
> >
> > _______________________________________________
> > xfs mailing list
> > xfs@oss.sgi.com
> > http://oss.sgi.com/mailman/listinfo/xfs