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# Blosc2 Sparse Frame Format

Blosc (as of version 2.0.0) has a Sparse Frame (SFrame for short) format that allows for non-contiguous storage of Blosc2 data chunks on disk.

When creating an sparse frame one must denote the contiguous flag in storage struct as false and provide a name (which represents a directory, but in the future it could be an arbitrary URL) in storage.urlpath for the sframe to be stored. It is recommended to name the directory with the .b2frame (or .b2f for short) extension.

A SFrame is made up of a frame index file and the chunks stored in the same directory on-disk. The frame index file follows the format described in the contiguous frame format document, with the difference that the frame’s chunks section is made up of multiple files (one per chunk). The frame index file name is always chunks.b2frame, and it also contains the metadata for the sframe.

## Chunks

The chunks are stored in the directory as binary files. Each chunk file name will be composed of the index of the chunk in hexadecimal written in capital letters with a length of 8 characters padded with zeros with the .chunk extension. As an example, 15 chunks could be named as follows:

    00000000.chunk, 00000001.chunk, ··· , 0000000E.chunk, 0000000F.chunk

Each chunk follows the format described in the chunk format document.

Note: The real order of the chunks is in the index chunk and may not follow the order of the names. This can occur when doing an insertion or a reorder. For more information see the Examples section below.

## Examples

### Structure example

As shown below, an sframe of 4 chunks will be composed of a directory with each chunk file and the frame file:

    dir.b2frame/
    │
    ├── 00000000.chunk
    │
    ├── 00000001.chunk
    │
    ├── 00000002.chunk
    │
    ├── 00000003.chunk
    │
    └── chunks.b2frame

### Insertion example

When doing an insertion in the nth position, in the same position of the index chunk will be the real chunk index which will be the numbers of chunks that there were before inserting the new one. Following the previous example, it its shown the content of the directory and the index chunk before and after an insertion in the 2nd position:

    Before                                 After

    dir.b2frame/                          dir.b2frame/
    │                                      │
    ├── 00000000.chunk                     ├── 00000000.chunk
    │                                      │
    ├── 00000001.chunk                     ├── 00000001.chunk
    │                                      │
    ├── 00000002.chunk                     ├── 00000002.chunk
    │                                      │
    ├── 00000003.chunk                     ├── 00000003.chunk
    │                                      │
    └── chunks.b2frame                     ├── 00000004.chunk
                                           │
                                           └── chunks.b2frame

    Possible index                         New index
    chunk content:  [0, 1, 2, 3]           chunk content: [0, 1, 4, 2, 3]

Note that neither the file names nor their contents change, so when accessing the 2nd chunk the 00000004.chunk file will be read.

### Reorder example

As in the insertion case, when doing a reorder the chunks names and their contents are not changed, but the content of the index chunk does. When reordering the chunks, a new order list is passed and the index chunk is changed according to that list. Following with the first example of this section, the content of the index chunk is shown before and after reordering:

    Before                                 After

    Possible index                         New index
    chunk content:  [0, 1, 2, 3]           chunk content:  [3, 1, 0, 2]
    New order list: [3, 1, 0, 2]