[Ed Hone (EdHone)]

PR Summary

Sci/Tech Reviewer: Mike Hobson (@mike-hobson)
Code Reviewer: svadams (@svadams)

The checkpointing I/O system used by some applications in lfric_apps uses 'legacy' checkpointing domains which are extremely non-performance at scale. The aim is convert to using the same UGRID format for output as the diagnostic files, which would involve changing the way that fields on the W2 function space are set up for I/O in XIOS. However, progress has been difficult for implementing this goal, as the UGRID implementation inside XIOS has some requirements and bugs which we cannot circumvent at the required timescales.
To get a performant checkpointing system working as soon as possible, this PR enables these applications to use checkpoint files to use a layered CF I/O implementation, which will circumvent the bugs in UGRID whilst maintaining the improved performance gains.

• linked Convert checkpoint files from legacy domains to layered CF lfric_apps#782

Code Quality Checklist

• I have performed a self-review of my own code
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Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
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trac.log

Test Suite Results - lfric_core - lfric_core-ugrid-ckp-flag/run3

Suite Information

Item	Value
Suite Name	lfric_core-ugrid-ckp-flag/run3
Suite User	edward.hone
Workflow Start	2026-09-30T14:22:35
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	EdHone/lfric_core@ugrid-ckp-flag	False
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True

Task Information

✅ succeeded tasks - 433

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Sci/Tech Review

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Code Review

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[Mike Hobson (mike-hobson)]

This change looks fine - just a couple of very minor points. Obviously, I've looked at the linked ticket for context. I'm a little concerned about the way we are controling legacy mode. Currently, legacy mode is selected if the run has a planar, biperiodic mesh. This allows the SCM runs to use legacy mode.

When we move to XIOS 3, the SCM checkpointing should work in layered mode. We will then want to remove the if-tests that automaticaaly select legacy mode. At that point, we will no longer be able to select legacy mode, so we'll have dead code. To avoid getting into this situation, I think we have a couple of options:

1. Given that XIOS 3 is very close, we could remove the support for legacy mode, in this change. That would mean this would have to wait until after XIOS 3 is done, but putting all this code in, so it can be removed in weeks seems wasteful and prone to error.
2. Introduce a namelist switch to turn legacy checkpointing on, then set the switch to "on" for the SCM jobs. That way, when the SCM runs work with layered checkpointing, it would be easy to switch them, and if we ever wanted to make any run use legacy checkpointing in the future, that would also be really easy.

I probably edge towards the second option. I know it wouldn't be particulalrly easy, as you have to pass the switch variable through the interface to the procedure pointer, but in my many experiments into checkpointing, I have already done this. If you'd like to see what I did:
https://github.com/mike-hobson/lfric_apps/tree/apps_layered
and
https://github.com/mike-hobson/lfric_core/tree/core_layered

What do you think?

[Mike Hobson (mike-hobson)]

Looks like you never implemented whatever you added imdi for. I think it can just be removed.

[Mike Hobson (mike-hobson)]

I realise that it's unlikely that these if-tests wouldn't catch all cases, but it would still be good to have an else option to catch any potential errors, rather than let the code continue without setting the file convention.