Keep ext4 journal on another system, how much space would be necessary?
$ man mkfs.ext4 The size of the journal must be at least 1024 filesystem blocks (i.e., 1MB if using 1k blocks, 4MB if using 4k blocks, etc.) and may be no more than 102,400 filesystem blocks.
I assume the default dimension is 128MB yet not exactly sure, that could be dated. Anyways I do not assume relocating journal to an additional dividing on the very same HDD will certainly be a renovation. If you relocate to an additional physical disk that can aid.
The most effective you can do is to attempt various dimensions and also contrast to existing state with your actual work (not some benchmark device that might or might not imitate procedures comparable to your actual work).