{
  "status": "shipping",
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-23",
  "advantages": [],
  "limitations": [],
  "slug": "zarr-fill-value",
  "term": "Zarr fill value",
  "aliases": [],
  "definition": "A Zarr fill value is the array-metadata value used to represent uninitialized elements, including absent chunk data under the format's read semantics.",
  "note": "Interpretation: a storage fill value is not automatically a scientific missing-value mask. Consumer decoding can treat that value as ordinary data or map it to a missing marker. [zarr-core] [xarray-open]\n\nElision: omitting a chunk whose contents are entirely fill can be an intentional storage optimization. A never-written chunk and a previously written then deleted chunk can leave the same final store, so absence needs an external completeness expectation. [hesela-zarr]\n\nStrict reads: Zarr-Python 3.4.1 with effective read_missing_chunks=False rejected missing selected store objects in Hesela's tests, but accepted empty inner chunks in a present shard. Verify effective runtime configuration and intended sparsity separately. [zarr-config] [zarr-sharding] [hesela-zarr]\n\nConsumer scope: the measured Xarray reduction results cover 2025.10.1 and 2026.9.0 over Zarr 3.1.3, not all readers or current versions. [hesela-zarr]",
  "related": [
    "zarr",
    "zarr-consolidated-metadata",
    "hdf5-fill-value"
  ],
  "citations": [
    {
      "id": "zarr-core",
      "title": "Zarr format 3 core specification: fill values and array reads",
      "url": "https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html",
      "accessed": "2026-10-23"
    },
    {
      "id": "zarr-sharding",
      "title": "Zarr sharding codec 1.0: empty inner chunks and index layout",
      "url": "https://zarr-specs.readthedocs.io/en/latest/v3/codecs/sharding-indexed/index.html",
      "accessed": "2026-10-23"
    },
    {
      "id": "zarr-config",
      "title": "Zarr-Python: runtime array configuration",
      "url": "https://zarr.readthedocs.io/en/latest/user-guide/arrays/#runtime-configuration",
      "accessed": "2026-10-23"
    },
    {
      "id": "xarray-open",
      "title": "Xarray 2025.10.1: open_zarr decoding contract",
      "url": "https://docs.xarray.dev/en/v2025.10.1/generated/xarray.open_zarr.html",
      "accessed": "2026-10-23"
    },
    {
      "id": "hesela-zarr",
      "title": "Hesela: Zarr archive acceptance observations",
      "url": "https://hesela.dev/datasets/zarr-archive-acceptance/",
      "accessed": "2026-10-23"
    }
  ]
}