I used gdisk (from SysRescue) to partition the disk.  Partition #1 of each device is a 200 (201?) MB EFI System Partition (I believe starting at sector 2048).

A NVMe "disk" is the root device, which has a bunch of partitions to be used eventually, and a scratch partition to install Devuan into initially.  If the previous partition is less than 1G, no extra space was inserted.  For partitions bigger than 1G, a space of 129 (or 128) MB was used.  There will be a bunch of partitions, and about half of the NVMe was partitioned.

The 4 (spinning rust) disks got a 16G partition (2 for swap, 2 for /tmp as a RAID-0), and then almost everything else to be part of a RAID-10 /home.

With gdisk, if you want an "empty space" before a partition begins, instead of using a sector number, you can give it an offset.  For a beginning value, the offset probably is positive.  So I typically used +129M as an offset (gdisk seems to round things, and if you want a 128MB space, it seems like you need to enter +129M).  For the end of disks, I wanted to assign all the space except for about 40GB or so, so I used -40G (or -48G ?) for the end value of a partition.

The thinking is that if you go to unfragment a partition at some point, and you are using a smart program for unfragmenting, it will notice this unallocated space adjoining the partition and use it when unfragmenting.

I like the idea of swap (I've almost killed VAX 11/785 computers with swapping issues way back when) and a big /tmp on spinning rust.  And from what I've read, having swap on SSD isn't really a good idea.

I have four 2TB Seagate NAS drives in the new machine.  The idea is to use a gpt partitioning on all of these devices, and set aside the first 32GB of each device for something.  On the first two disks, they get set aside for (parallel) swap.  On the second two disks, they get set aside to be a RAID-0 (striped) /tmp (ext2).

So that leaves us with almost 2TB of free space on all devices (and nominally all the same size).

I think (no proof) that leaving some space unallocated is a good thing.  I'm currently playing with the Debian-Buster alpha2 installer (as it boots) on the new hardware, and it has a limited idea of how to do gpt.  It will allow me to place a partition at the beginning or end of free space.  It will not (easily) allow me to put a partition in the middle of free space.

Does a person need a bit of unallocated space at the end of a swap or /tmp partition?  If a person doesn't need unallocated space at the end of those partitions, then installing a partition onto the beginning of the free space following that swap (or /tmp) should work fine.  I was thinking of using 98% of the free space, which would leave something like 40GB at the end of the disk unallocated.  Is that enough?  Or should a person only allocate say 70%, so that you can free up some room for storage when you go to buy new disks and are waiting for them to get delivered?