v7.3 mat file compression

I wanted to check if there is still no way to control compression in the new v7.3 HDF mat file format.

I have found things on the web that say only the first variable saved is compressed, and using append then saves the additional variables without compression.

Is this obscure trick really the only way to control it? I don't see anything in the documentation. I would like to use the v7.3 format for big data but I would really want to control use of compression on a per file (rather than per variable) basis.

Are additional variables not compressed even when saving to the file all at once? Is it possible to have multiple compressed variables in a file?

This seems such core functionality I am amazed that I cannot find any consistent answers.

Answer 1

[Robin]

I found bug 784028 which suggests the append behaviour is "fixed" in 2012a.

http://www.mathworks.co.uk/support/bugreports/784028

But many questions remain. Is it now impossible to save uncompressed data with -v7.3? (since previously the only way was by exploiting the append bug).

What is the criteria for whether a variable is compressed? There seems to be some size test - small variables are not compressed.

EDIT:

Some tedious trial and error using the third party HDFView tool apparently indicates that if the variable size is > 10000 bytes compression if used and not otherwise.