Re: [PATCH v5 11/24] xfs: add XBF_VERITY_SEEN xfs_buf flag

Darrick J. Wong
2024-03-12T04:15:57Z

On Mon, Mar 11, 2024 at 07:43:20PM -0700, Eric Biggers wrote:
> On Mon, Mar 11, 2024 at 08:25:05AM -0700, Darrick J. Wong wrote:
> > > > > I wonder, what are the access patterns for merkle blobs? Is it actually
> > > > > sequential, or is more like 0 -> N -> N*N as we walk towards leaves?
> > >
> > > I think the leaf level (i.e. individual record) access patterns
> > > largely match data access patterns, so I'd just treat it like as if
> > > it's a normal file being accessed....
> >
> > <nod> The latest version of this tries to avoid letting reclaim take the
> > top of the tree. Logically this makes sense to me to reduce read verify
> > latency, but I was hoping Eric or Andrey or someone with more
> > familiarity with fsverity would chime in on whether or not that made
> > sense.
>
> The levels are stored ordered from root to leaf; they aren't interleaved. So
> technically the overall access pattern isn't sequential, but it is sequential
> within each level, and the leaf level is over 99% of the accesses anyway
> (assuming the default parameters where each block has 128 children).

<nod>

> It makes sense to try to keep the top levels, i.e. the blocks with the lowest
> indices, cached. (The other filesystems that support fsverity currently aren't
> doing anything special to treat them differently from other pagecache pages.)

<nod>

--D

> - Eric
>