I have usb keychain with size of 7.5GB and I need to copy file on it with size 7.4GB. But I can't because superblocks consume 0.5GB of space.
You don't need a file system to write data to a device. You can simply use
You can directly use
In my experiments this always reads the entire block device, not just the data in the tar archive. If you know that your device has 8 GiB but you only saved, say 3 GiB, you can use
Try to compress the data as much as possible. This might take a long time, but maybe everything fits on a drive with an ordinary filesystem. I would advice to use
Since you don't really need an ext4 file system but are really asking about a file system that will bring the overhead down below about 1.3% of the device capacity (100 MB out of 7.5 GB), I'd look at various low-overhead options. The two most obvious that meet your criteria of being able to handle a single 7.4 GB file is either ext2 with a low inode count and sparse superblocks, or a low-overhead FAT32 file system.
EDIT: It looks like I was wrong about FAT32's maximum file size, but I'm leaving it here in case someone comes across this and can live with the limitation that a single file cannot be larger than 2^32 - 1 (4 GiB - 1) bytes.
For a low-overhead ext2 file system, try something along the lines of
For a low-overhead FAT32 file system, try something like