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How to create a proper size UDF image?

Hi all,

If we'd like to burn some data files using UDF file system, normally we should firstly make a UDF loop file and copy files into it.
The size of UDF file is a headache because it is not same with the size given by du. Actually the du size is calculated by the file system which the files to be burned are located. In some conditions, the two sizes are different very large. For example I created about 150K zero-size files using touch on the ext4 file system. The du size is only 3.5MB compared with 300MB udf file size.

On the other size, it is not acceptable to create a udf file with the size of media disc. The empty session not used in the file can increase the burning time.

I know that some tools like mkisofs could make files to a udf image directly with efficient size. However the size created by mkisofs could be larger than the media size. What's more, the udf filesystem made by mkisofs is a UDF hybrid and not a UDF only filesystem.   

So is there a rational method to get the rough size of a UDF image file to be created if we know the directory tree and sizes of files needed to be burned? I think 10% larger size  overhead could be acceptable. 


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