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

Brian Foster
2016-08-02T21:38:27Z

On Tue, Aug 02, 2016 at 01:06:35PM -0700, Darrick J. Wong wrote:
> 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
>

Thanks! Seems Ok to me.. with that in the commit log:

Reviewed-by: Brian Foster <bfoster@redhat.com>

> 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 */
> > >
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> > > xfs@oss.sgi.com
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