Behavior

• ✅ Works: filename = "output" → creates output_rank0.jld2, output_rank1.jld2, etc.
• ❌ Fails: filename = "output.jld2" → hangs (all ranks attempt to write to same file)

Reproducer

using MPI, Oceananigans
MPI.Init()

arch = Distributed(GPU(); partition = Partition(2, 2))
grid = RectilinearGrid(arch, size=(64, 64, 16), extent=(1, 1, 1))
model = NonhydrostaticModel(; grid)
simulation = Simulation(model, Δt=10, stop_iteration=100)

# This hangs:
simulation.output_writers[:out] = JLD2Writer(model, model.tracers,
    filename = "output.jld2",
    schedule = IterationInterval(10))

run!(simulation)

Test Results

Tested with filename = "output" (without extension):

• Grid: 1440×720×30 (0.25° Arctic Ocean)
• GPUs: 4× RTX 4090
• Partition: Partition(2, 2)
• Output interval: 6 hours

Results:

• ✅ All 4 ranks completed successfully
• ✅ Created output_rank0.jld2 through output_rank3.jld2
• ✅ Good I/O performance (~10ms write time after initialization)

Originally reported in ClimaOcean.jl: CliMA/ClimaOcean.jl#746
Credit to @glwagner for diagnosing the bug.