MAT File Versions
Overview of MAT File Versions
MAT files are binary MATLAB® files that store workspace variables. Starting with MAT file Version 4, there are several subsequent versions of MAT files that support an increasing set of features. MATLAB releases R2006b and later all support all MAT file versions.
By default, all save operations create Version 7 MAT files. The only exception to
this is when you create new MAT files using the matfile
function. In this case, the default MAT file version is 7.3.
To identify or change the default MAT file version, access the MAT and FIG Files settings.
On the Home tab, in the Environment section, click Settings.
Select MATLAB > General > MAT and FIG Files.
The settings apply to both the save function and the
Save menu options.
The maximum size of a MAT file is imposed only by your native file system.
This table lists and compares all MAT file versions.
| MAT File Version | Supported MATLAB Releases | Supported Features | Compression | Maximum Size of Each Variable | Value of version argument in
save function | Setting Option |
|---|---|---|---|---|---|---|
| Version 7.3 | R2006b (Version 7.3) or later | Saving and loading parts of variables, and all Version 7 features | Yes (default) | ≥ 2 GB on 64-bit computers | "-v7.3" | MATLAB Version 7.3 or later (save -v7.3) |
| Version 7 | R14 (Version 7.0) or later | Unicode® character encoding, which enables file sharing between systems that use different default character encoding schemes, and all Version 6 features. | Yes (default) | 231 bytes per variable | "-v7" | MATLAB Version 7 or later (save -v7) |
| Version 6 | R8 (Version 5) or later |
| No | 231 bytes per variable | "-v6" | MATLAB Version 5 or later (save -v6) |
| Version 4 | All | Two-dimensional | No | 100,000,000 elements per array, and 231 bytes per variable | "-v4" | n/a |
Note
Version 7.3 MAT files use an HDF5 based format that requires some overhead storage to describe the contents of the file. For cell arrays, structure arrays, or other containers that can store heterogeneous data types, Version 7.3 MAT files are sometimes larger than Version 7 MAT files.
Save to Nondefault MAT File Version
Save to a MAT file version other than the default version when you want to:
Allow access to the file using earlier versions of MATLAB.
Take advantage of Version 7.3 MAT file features.
Reduce the time required to load and save some files by storing uncompressed data.
Reduce the size of some files by storing compressed data.
To save to a MAT file version other than the default version, specify the
version argument as the last input to the save function. For example, to
create a Version 6 MAT file named myfile.mat, type:
save("myfile.mat","-v6")
Data Compression
Beginning with Version 7, MATLAB compresses data when writing to MAT files to save storage space. Data compression and decompression slow down all save operations and some load operations. In most cases, the reduction in file size is worth the additional time spent.
In some cases, loading compressed data actually can be faster than loading uncompressed data. For example, consider a block of data in a numeric array saved to both a 10 MB compressed file and a 100 MB uncompressed file. Loading the first 10 MB takes the same amount of time for each file. Loading the remaining 90 MB from the uncompressed file takes nine times as long as loading the first 10 MB. Completing the load of the compressed file requires only the relatively short time to decompress the data.
The benefits of data compression are negligible in the following cases:
The amount of data in each item is small relative to the complexity of its container. For example, simple numeric arrays take less time to compress and uncompress than cell or structure arrays of the same size. Compressing arrays that result in an uncompressed file size of less than 3 MB offers limited benefit, unless you are transferring data over a network.
The data is random, with no repeated patterns or consistent values.
Accelerate Save and Load Operations for Version 7.3 MAT Files
Version 7.3 MAT files use an HDF5-based format that stores data in compressed chunks. The time required to load part of a variable from a Version 7.3 MAT file depends on how that data is stored across one or more chunks. Each chunk that contains any portion of the data you want to load must be fully uncompressed to access the data. Rechunking your data can improve the performance of the load operation. To rechunk data, use the HDF5 command-line tools, which are part of the HDF5 distribution.
