Blosc codec (version 1.0)#

Editor’s draft 26 July 2019

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Corresponding ZEP:

ZEP0001 — Zarr specification version 3

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Copyright 2020-Present Zarr core development team. This work is licensed under a Creative Commons Attribution 3.0 Unported License.


Defines a bytes -> bytes codec that uses the blosc container format.

Status of this document#

ZEP0001 was accepted on May 15th, 2023 via zarr-developers/zarr-specs#227.

Document conventions#

Conformance requirements are expressed with a combination of descriptive assertions and [RFC2119] terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in [RFC2119]. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. Examples in this specification are introduced with the words “for example”.

Codec name#

The value of the name member in the codec object MUST be blosc.

Configuration parameters#


A string identifying the internal compression algorithm to be used. At the time of writing, the following values are supported by the c-blosc library: “lz4”, “lz4hc”, “blosclz”, “zstd”, “snappy”, “zlib”.


An integer from 0 to 9 which controls the speed and level of compression. A level of 1 is the fastest compression method and produces the least compressions, while 9 is slowest and produces the most compression. Compression is turned off completely when level is 0.


Specifies the type of shuffling to perform, if any, prior to compression. Must be one of:

  • "noshuffle", to indicate no shuffling;

  • "shuffle", to indicate byte-wise shuffling;

  • "bitshuffle", to indicate bit-wise shuffling.

Zarr implementations MAY provide users an option to choose a shuffle mode automatically based on the typesize or other information, but MUST record in the metadata the mode that is chosen.


Positive integer specifying the stride in bytes over which shuffling is performed. Required unless shuffle is "noshuffle", in which case the value is ignored.

Zarr implementations MAY allow users to leave this unspecified and have the implementation choose a value automatically based on the array data type and previous codecs in the chain, but MUST record in the metadata the value that is chosen.


An integer giving the size in bytes of blocks into which a buffer is divided before compression. A value of 0 indicates that an automatic size will be used.

For example, the array metadata document below specifies that the compressor is the Blosc codec configured with a compression level of 1, byte-wise shuffling with a stride of 4, the lz4 compression algorithm and the default block size:

    "codecs": [{
        "name": "blosc",
        "configuration": {
            "cname": "lz4",
            "clevel": 1,
            "shuffle": "shuffle",
            "typesize": 4,
            "blocksize": 0

Format and algorithm#

This is a bytes -> bytes codec.

Blosc is a meta-compressor, which divides an input buffer into blocks, then applies an internal compression algorithm to each block, then packs the encoded blocks together into a single output buffer with a header. The format of the encoded buffer is defined in [BLOSC]. The reference implementation is provided by the c-blosc library.

Comparison to Zarr v2#

While the binary format is identical, the JSON metadata differs from that used by the Zarr v2 blosc codec in the following ways:

  • The shuffle mode is now specified more clearly as noshuffle (0 in Zarr v2), “bitshuffle” (2 in Zarr v2), or “shuffle” (1 in Zarr v2). Using these constants rather than numbers makes it much easier to know what shuffle mode will be used from manual inspection of the metadata.

  • When shuffling is enabled, the typesize must now be specified explicitly in the metadata, rather than determined implicitly from the input data. This allows Blosc to function as a pure “bytes -> bytes” codec rather than an “array -> bytes” codec.

  • There is no option to choose between bit-wise and byte-wise shuffling automatically, as supported in Zarr v2 via a shuffle value of -1.



S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:


F. Alted. Blosc Chunk Format. URL: Blosc/c-blosc

Change log#

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