Hi again @HarryHUK,
Before some Btrfs experts provide more information here, I can chip in a little bit, especially with regards to the following:
You might be referring to the “recently”-added support in Btrfs for different raid levels between data and metadata combined to raid levels raid1c3 and raid1c4, that is particularly interesting for those wanting to run raid5/6 on their data. Indeed, it would allow you to have better redundancy for your metadata while keeping your data in raid5/6. We have had some quick discussion on implementing this in the forum (see the post linked below, for instance):
and @phillxnet’s response:
As you can see, this is something that will most likely be implemented Rockstor once it is made fully available in our base distribution as that will most likely match with when the openSUSE folks deem the feature ready and stable enough. As far as I know, this hasn’t been backported to Leap 15.2 yet so I don’t think it is ready yet.
On a side note, from the discussions I’ve read online, it seems that filesystem developers in general (not just Btrfs) seem to focus primarily on raid1/10 nowadays so raid5/6 might not be their primary focus. You can see quite a bit of work and discussion on raid5/6 on the Btrfs mailing list, though, so it has seen big improvements recently related to speed and reliability, I believe. Operations like scrub in Raid5/6 were famous for being very slow in older kernels when compared to raid1/10, for instance, and that has improved, for instance. Disabling quotas is also recommended to improve speed/performance.
I know I haven’t really answered your question(s), but I hope I was able to bring some elements of answer nonetheless!