Enlighten us by elaborating on why that is a terrible idea, because I am either too dumb to grasp it or you are wrong.
Mirroring of 2 disks can tolerate the failure of one disk. If it were 4 physical disks, you would also choose striped mirrors as well, which gives the same fault tolerance. In the specific case of 8+4+4, there could be even two disks (i.e. sdb and sdc) failing without any data loss, so I fail to see what you mean. Obviously, if sda fails, the fault tolerance is still one disk, so not worse than with 2 or 4 disks.
The only thing you would not get is a speedup by only one stripe of 4+4, but you would not get that with two 8 TByte disks, either.
So, I challenge you (mostly because I want to learn): Why is that a terrible idea?
Yet you fail to substantiate that it is stupid beyond you saying it was so. As I said, that is your opinion.LOL what reality do you live in where "the burden of proof is on me"? I'm trying to warn OP against doing something stupid.
I would A/ get a extra 4tb drive then you can have a raidx1 8tb or B/ get a extra 8tb so you can have 4tb + 8tb mirrors , but I guess that wasn't the question asked .Probably, ZFS will try to distribute data over different vdevs, such that the two partitions on the 8 TB Disk will be used in turns, causing seeks. This would not matter on SSDs. I think the size of the Write Blocks would matter, so to minimize the seeks, I would use a larger ashift than usual.
Trying to think how many years ago it was 20 - 25 year , I was into Ham packet radio 70cm / 2m / 4m which is slightly faster than IP over carrier pigeon , but I think the internet killed packet radio too.With Linux you have the freedom and the capability to do a very lot of things, including weird ones.
Yes, you can (for example) use the LVM to create a Volume Group consisting of two 4 GB disks as Physical Volumes to get a 8 GB Logical Volume. And yes, a "zpool create mypool mirror xxx yyy" would accept that.
I did not test this I never would. But please: do so! And please report back your experience after the first disk failures have occurred. (Not speaking of performance as that is probably secondary...)
But really... not everything what's possibly is a good idea!
To access the internet you can run https://en.wikipedia.org/wiki/IP_over_Avian_Carriers (https://datatracker.ietf.org/doc/html/rfc1149) but I would nobody expect to do so![]()