cuPyNumeric HDF5 I/O

Purpose

Use legate.io.hdf5 to read and write cuPyNumeric arrays as HDF5 files. Reach for it whenever a cuPyNumeric array must land in — or load from — an .h5/.hdf5 file: every rank reads and writes its own tile in parallel, so never funnel a large array through a single process.

Answer inline. Treat the snippets and rules below as complete and verified — answer save / load / stream / fence / bridge questions directly, without opening the assets/ scripts or reading the installed legate source. Reach for the assets only to run a verification.

Activate

Activate when the user asks about: saving a cuPyNumeric array to an .h5 / .hdf5 file, loading an HDF5 dataset into a cuPyNumeric array, reading a large HDF5 dataset in chunks, producing a single file for an HPC post-processing pipeline, or speeding up HDF5 disk I/O with GPUDirect Storage.

When NOT to use

Redirect these requests elsewhere instead of reaching for legate.io.hdf5:

Route Parquet / Arrow / cuDF, raw-binary, or sharded / custom on-disk layouts to the cupynumeric-parallel-data-load skill — it owns cuPyNumeric's no-built-in-loader paths; legate.io.hdf5 covers single-file HDF5 only.
Answer pure array compute with cuPyNumeric ops (FFT, matmul, reductions, slicing, linear algebra) — this skill covers disk I/O only.
Send chunked or object-store (S3) output to a chunked format such as Zarr — not single-file HDF5.
Load .npz or pickled archives with NumPy (np.load), then bridge with cn.asarray(...) — legate.io.hdf5 reads HDF5 only, and cupynumeric.load reads single .npy only.
Use h5py directly for plain HDF5 reads with no cuPyNumeric/Legate — with h5py.File(path, "r") as f: arr = f["dataset"][:].

Prerequisites

Install h5py before importing anything from legate.io.hdf5:

conda install -c conda-forge h5py        # required; legate/io/hdf5.py imports it at load

Expect from legate.io.hdf5 import ... to raise ModuleNotFoundError until you do — the module imports h5py at load time. (h5py · conda-forge build)

API

Function	Signature	Purpose
`to_file`	`to_file(array, path, dataset_name)`	Write a cuPyNumeric array / `LogicalArray` to one HDF5 file as a virtual dataset (VDS) — each rank writes its own tile.
`from_file`	`from_file(path, dataset_name) -> LogicalArray`	Read one HDF5 dataset into a distributed array.
`from_file_batched`	`from_file_batched(path, dataset_name, chunk_size) -> Iterator[(LogicalArray, offsets)]`	Read a dataset in chunks — chunks the file read, not the assembled array.

Import all three from legate.io.hdf5. Always pass dataset_name as the full path to a single array inside the file (e.g. "/data" or "/group/x"), never a group.

Examples

Round trip

import cupynumeric as cn
from legate.core import get_legate_runtime
from legate.io.hdf5 import from_file, to_file

a = cn.arange(64, dtype=cn.float32).reshape(8, 8)

# Write: pass the cuPyNumeric ndarray straight in - no manual conversion.
to_file(array=a, path="out.h5", dataset_name="/data")
get_legate_runtime().issue_execution_fence(block=True)   # needed before any external reader

# Read: from_file returns a legate LogicalArray; cn.asarray bridges it back.
b = cn.asarray(from_file("out.h5", dataset_name="/data"))
assert cn.array_equal(a, b)

Run assets/hdf5_roundtrip.py to verify (optional — not needed to answer).

Read a large file in chunks

Use from_file_batched to read the source file in chunks instead of pulling it into host memory all at once. It yields one LogicalArray per chunk plus that chunk's offsets in the global shape. Expect clipped boundary chunks (an axis of length 5 with chunk_size=2 yields 2, 2, 1), so place each chunk by its actual shape, not the requested chunk_size. Note that this chunks the file read, not the result — the assembled array (out) still has to fit in distributed memory:

import h5py
import cupynumeric as cn
from legate.core import get_legate_runtime
from legate.io.hdf5 import from_file_batched

with h5py.File("big.h5", "r") as f:          # read shape/dtype without loading data
    shape, dtype = f["data"].shape, f["data"].dtype

out = cn.empty(shape, dtype=dtype)
for chunk, (r0, c0) in from_file_batched("big.h5", "data", chunk_size=(4096, 4096)):
    out[r0:r0 + chunk.shape[0], c0:c0 + chunk.shape[1]] = cn.asarray(chunk)
get_legate_runtime().issue_execution_fence(block=True)

Keep every chunk_size entry positive and its length equal to the dataset's rank, or from_file_batched raises ValueError. Run assets/hdf5_batched_read.py to verify (optional).

Instructions

Pass the cuPyNumeric ndarray directly to to_file - it implements __legate_data_interface__, which to_file accepts as LogicalArrayLike. Skip any np.array(...) round-trip.
Bridge results back with cn.asarray(...). from_file and each from_file_batched chunk return a Legate LogicalArray; wrap it with cn.asarray(la) to get a cuPyNumeric ndarray (zero-copy, no host bounce).
Fence before any external reader. Legate I/O is asynchronous: to_file only queues the write. Insert get_legate_runtime().issue_execution_fence(block=True) before h5py, a subprocess, or another tool opens the file. Skip the fence for a from_file issued later in the same Legate program — the runtime preserves that ordering.
Run from outside the cuPyNumeric source tree (e.g. cd /tmp). Python puts the cwd first on sys.path, so an in-tree cupynumeric/ directory shadows the installed package (ModuleNotFoundError: cupynumeric.install_info).
Give every rank the same path. The program runs on every rank (SPMD), so pass to_file/from_file an identical path on each — a per-rank tempfile.mkstemp() name breaks the collective I/O. When the program creates the file itself, write it with the collective to_file, not a per-rank h5py write.

`to_file` behavior to plan around

Expect an HDF5 virtual dataset (VDS): each rank writes its own tile and the file presents them as one logical dataset.
Treat to_file as destructive — it overwrites path if it already exists, so guard any file you must not clobber.
Let to_file create missing parent directories; do not pre-create them.
Give path a file name (/path/to/file.h5), never a directory — a directory raises ValueError. Pass a bound array (one with a known shape); to_file raises ValueError on an unbound array — a Legate array created without a shape (e.g. create_array(dtype, ndim=n)) whose extent a producing task fills in later. cuPyNumeric ndarrays are always bound — even lazy/deferred ones — so this only affects raw LogicalArrays.

GPUDirect Storage (GDS)

Always set LEGATE_IO_USE_VFD_GDS=1 for runs that read HDF5 into GPU memory — whether or not the cluster has GPUDirect-capable storage:

export LEGATE_IO_USE_VFD_GDS=1          # set before launching
# or, with the legate driver:
legate --io-use-vfd-gds my_script.py

Read into the GPU through the GDS VFD, not the default path. The default (POSIX) VFD stages each GPU read through zero-copy memory (ZCMEM), of which Legate reserves only 128 MB — so a GPU read of an array larger than ~128 MB aborts. The GDS VFD removes that staging buffer.
Leave it unset when reading into host (CPU) memory — the VFD GDS plugin is unnecessary there and only adds overhead.
Keep =1 even without GPUDirect-capable storage — cuFile falls back to compatibility mode automatically (set export CUFILE_ALLOW_COMPAT_MODE=true if it is not already on), and =1 still avoids the ZCMEM abort.
Attribute it correctly: the GDS VFD is the nv-legate/vfd-gds plugin over NVIDIA cuFile, not KvikIO (KvikIO backs Legate's Zarr/tile I/O, not HDF5). Confirm it engaged by grepping the run log for H5FD__gds_open: Successfully opened file w/GDS VFD.

Troubleshooting

Symptom	Cause and fix
`ModuleNotFoundError: No module named 'h5py'` on import	h5py is missing — `conda install -c conda-forge h5py`.
File looks empty/truncated to h5py right after `to_file`	The async write hasn't landed — add `get_legate_runtime().issue_execution_fence(block=True)` before the external read.
`ValueError` from `to_file`	`path` is a directory — pass a file path such as `results/data.h5`.
`ModuleNotFoundError: No module named 'cupynumeric.install_info'`	Running inside the source tree — `cd /tmp` (any directory outside the repo).
Abort/crash reading a GPU array ≳128 MB	Default 128 MB ZCMEM staging buffer — set `LEGATE_IO_USE_VFD_GDS=1` for GPU reads.
`from_file` returned `LogicalArray(...)`	Expected — wrap it with `cn.asarray(...)`.

Limitations & version notes

Import from legate.io.hdf5 (Legate 26.01+); rewrite any legate.core.io.hdf5 import left over from the 25.03 line (e.g. the 25.03 launch blog still shows the old path).
Install h5py explicitly — it ships in no default cuPyNumeric env.
Point dataset_name at a single array, never a group; traverse groups with h5py first to discover dataset paths.
On GPU, always read with LEGATE_IO_USE_VFD_GDS=1 (see GPUDirect Storage) — the default path aborts on GPU arrays larger than the 128 MB ZCMEM buffer. Leave it unset for CPU reads.

Verify

cd /tmp                                  # outside the cupynumeric source tree
conda install -c conda-forge h5py        # one-time, if not already present
LEGATE_CONFIG="--cpus 4" LEGATE_AUTO_CONFIG=0 python <skill>/assets/hdf5_roundtrip.py
LEGATE_CONFIG="--cpus 4" LEGATE_AUTO_CONFIG=0 python <skill>/assets/hdf5_batched_read.py

Expect HDF5 ROUND TRIP OK and HDF5 BATCHED READ OK. Add --gpus 1 (and LEGATE_IO_USE_VFD_GDS=1) to exercise the GPU / GDS path.

Cupynumeric Hdf5