In Ext4 if both flex_bg & meta_bg features are enabled, then the copies of super-block and group descriptors follow which pattern? Does it follow the sparse super-block concept ? Also in sparse super-block concept, the copies of super-block are kept in powers of 3 or 5 or 7 or in all the powers of 3 and 5 and 7 ?
I'm not an expert on this topic, but this sounds very much like a question for one of the kernel mailing lists.
From what I've read though, sparse super block puts the block to each of the powers, so that would boil down to block group: 0, 1, 3, 5, 7, 9, 25, 27, 49, 81, etc ...
The articles covered Ext2 on this, but I think that by extension this pattern should also apply to ext4.
Now, as I haven't looked into file system implementations yet, I honestly wouldn't know what those flex_bg and meta_bg flags would do.