genpkg padding for bin => zip files

[Paul]

I compile a .bin file for DFU. The bin file is 62020 bytes. I create the zip file for DFU, it is 62909 bytes.

If I extract the zip file, I get the bin file (62020 bytes), a json file (482 bytes), and a dat file (14 bytes). In other words the sum of the size of the files in the zip file is 62516 bytes which is smaller than the zip file (62909) bytes - where are the extra 393 bytes coming from?

As a rule, shouldn't the zip file be smaller than the sum of it's extracted contents?

[Hung Bui]

Hi Paul,

Well, it might not always be the case. If the compression mechanism can't do any reduction on the file size then the overhead (header) will cause the .zip file size bigger than the original files size. In this case the .bin file is pretty hard to compress.

There are some questions about that already asked.

The main reason we use a .zip file to distribute the image is to combine all in one single file, make it easier for end customer. It's not about compressing the size.

[Hung Bui]

Hi Paul,

You should check the actual maximum size for application by looking at nrf_dfu_find_cache() if you are using Secure DFU in SDK 12 and up. Or look for DFU_IMAGE_MAX_SIZE_BANKED if you are using SDK v11 and ealier.

What we send over the air is not the .zip file but the actual image (the binary) so it doesn't matter how big the .zip file is but how big the .bin file is.

Note that by default we will switch to single bank automatically (on Secure DFU) if file size larger than maximum for dual bank.