Re: [PATCH v5 09/10] xfs: Allow block allocator to take an alignment hint

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Mar 12, 2025 at 06:45:12AM -0700, Christoph Hellwig wrote:
> On Wed, Mar 12, 2025 at 08:05:14AM +0000, John Garry wrote:
> > > Shouldn't we be doing this by default for any extent size hint
> > > based allocations?
> > 
> > I'm not sure.
> > 
> > I think that currently users just expect extszhint to hint at the
> > granularity only.

Yes, the current behavior is that extszhint only affects the granularity
of the file range that's passed into the allocator.  To align the actual
space, you have to set the raid stripe parameters.

I can see how that sorta made sense in the old days -- the fs could get
moved between raid arrays (or the raid array gets reconfigured), so you
want the actual allocations to be aligned to whatever the current
hardware config advertises.  The extent size hint is merely a means to
amortize the cost of allocation/second-guess the delalloc machinery.

> > Maybe users don't require alignment and adding an alignment requirement just
> > leads to more fragmentation.
> 
> But does it?  Once an extsize hint is set I'd expect that we keep
> getting more allocation with it.  And keeping the aligned is the concept
> of a buddy allocator which reduces fragmentation.  Because of that I
> wonder why we aren't doing that by default.

Histerical raisins?

We /could/ let extszhint influence allocation alignment by default, but
then anyone who had (say) a 8k hint on a 32k raid stripe might be
surprised when the allocator behavior changes.

What do you say about logic like this?

	if (software_atomic) {
		/*
		 * align things so we can use hw atomic on the next
		 * overwrite, no matter what hw says
		 */
		args->alignment = ip->i_extsize;
	} else if (raid_stripe) {
		/* otherwise try to align for better raid performance */
		args->alignment = mp->m_dalign;
	} else if (ip->i_extsize) {
		/* if no raid, align to the hint provided */
		args->alignment = ip->i_extsize;
	} else {
		args->alignment = 1;
	}

Hm?  (I'm probably forgetting something...)

--D