On Tue, Dec 20, 2022 at 04:00:01PM +1100, Dave Chinner wrote:
> On Mon, Dec 19, 2022 at 04:05:19PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@xxxxxxxxxx>
> >
> > Lately I've been stress-testing extreme-sized rmap btrees by using the
> > (new) xfs_db bmap_inflate command to clone bmbt mappings billions of
> > times and then using xfs_repair to build new rmap and refcount btrees.
> > This of course is /much/ faster than actually FICLONEing a file billions
> > of times.
> >
> > Unfortunately, xfs_repair fails in xfs_btree_bload_compute_geometry with
> > EOVERFLOW, which indicates that xfs_mount.m_rmap_maxlevels is not
> > sufficiently large for the test scenario. For a 1TB filesystem (~67
> > million AG blocks, 4 AGs) the btheight command reports:
> >
> > $ xfs_db -c 'btheight -n 4400801200 -w min rmapbt' /dev/sda
> > rmapbt: worst case per 4096-byte block: 84 records (leaf) / 45 keyptrs (node)
> > level 0: 4400801200 records, 52390491 blocks
> > level 1: 52390491 records, 1164234 blocks
> > level 2: 1164234 records, 25872 blocks
> > level 3: 25872 records, 575 blocks
> > level 4: 575 records, 13 blocks
> > level 5: 13 records, 1 block
> > 6 levels, 53581186 blocks total
> >
> > The AG is sufficiently large to build this rmap btree. Unfortunately,
> > m_rmap_maxlevels is 5. Augmenting the loop in the space->height
> > function to report height, node blocks, and blocks remaining produces
> > this:
> >
> > ht 1 node_blocks 45 blockleft 67108863
> > ht 2 node_blocks 2025 blockleft 67108818
> > ht 3 node_blocks 91125 blockleft 67106793
> > ht 4 node_blocks 4100625 blockleft 67015668
> > final height: 5
> >
> > The goal of this function is to compute the maximum height btree that
> > can be stored in the given number of ondisk fsblocks. Starting with the
> > top level of the tree, each iteration through the loop adds the fanout
> > factor of the next level down until we run out of blocks. IOWs, maximum
> > height is achieved by using the smallest fanout factor that can apply
> > to that level.
> >
> > However, the loop setup is not correct. Top level btree blocks are
> > allowed to contain fewer than minrecs items, so the computation is
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> Ah, that's the critical piece of information I was looking for. I
> couldn't work out from the code change below what was wrong with
> limits[1]. So....
>
> > diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> > index 4c16c8c31fcb..8d11d3f5e529 100644
> > --- a/fs/xfs/libxfs/xfs_btree.c
> > +++ b/fs/xfs/libxfs/xfs_btree.c
> > @@ -4666,7 +4666,11 @@ xfs_btree_space_to_height(
> > const unsigned int *limits,
> > unsigned long long leaf_blocks)
> > {
> > - unsigned long long node_blocks = limits[1];
> > + /*
> > + * The root btree block can have a fanout between 2 and maxrecs because
> > + * the tree might not be big enough to fill it.
> > + */
>
> Can you change this comment to say something like:
>
> /*
> * The root btree block can have less than minrecs pointers
> * in it because the tree might not be big enough to require
> * that amount of fanout. Hence it has a minimum size of
> * 2 pointers, not limits[1].
> */
Done. Thanks for the reviews! :)
--D
>
> Otherwise it looks good.
>
> Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
>
> > + unsigned long long node_blocks = 2;
> > unsigned long long blocks_left = leaf_blocks - 1;
> > unsigned int height = 1;
>
> For future consideration, we don't use maxrecs in this calculation
> at all - should we just pass minrecs into the function rather than
> an array of limits?
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
