The thing that strike me most in your case is the partitioning. That is a disk might be used by more than one vdev (aka group), which is of course a major risk, but I understand perfectly that this is an old PC giving a new life as a NAS (I have one at home as well).

There is no major loophole in your schema, except that:

• a pool cannot be composed of different type of vdev: you cannot mix a 8-disks raidz and a 2-disks mirror into the same pool.
• raidz2 should be used starting with 6 disks. Either you divide your data pool into 2x 4-disks raidz or you go for 1x 8-disks raidz2
• you need to be extra-careful in assigning the disks into the vdev ;)

Here are some best practices (details here) regarding the building of groups:

• Start a single-parity RAIDZ (raidz) configuration at 3 disks (2+1)
• Start a double-parity RAIDZ (raidz2) configuration at 6 disks (4+2)
• Start a triple-parity RAIDZ (raidz3) configuration at 9 disks (6+3)
• The recommended number of disks per group is between 3 and 9. If you have more disks, use multiple groups (vdev).

If I may add, keep in mind as well that your system will eventually evolve. You might want to be able to replace disks by bigger ones which would only be possible if your group are not too big (or you'll end up in having to get 8 new disks before being able to get some more space from your data pool).

The thing that strike me most in your case is the partitioning. That is a disk might be used by more than one vdev (aka group), which is of course a major risk, but I understand perfectly that this is an old PC giving a new life as a NAS (I have one at home as well).

There is no major loophole in your schema, except that:

• a pool can be composed of different type of vdev but it is not recommended (at all)
• raidz2 should be used starting with 6 disks. Either you divide your data pool into 2x 4-disks raidz or you go for 1x 8-disks raidz2
• you need to be extra-careful in assigning the disks into the vdev ;)

Here are some best practices (details here) regarding the building of groups:

• Start a single-parity RAIDZ (raidz) configuration at 3 disks (2+1)
• Start a double-parity RAIDZ (raidz2) configuration at 6 disks (4+2)
• Start a triple-parity RAIDZ (raidz3) configuration at 9 disks (6+3)
• The recommended number of disks per group is between 3 and 9. If you have more disks, use multiple groups (vdev).

If I may add, keep in mind as well that your system will eventually evolve. You might want to be able to replace disks by bigger ones which would only be possible if your group are not too big (or you'll end up in having to get 8 new disks before being able to get some more space from your data pool).

You may as well consider forcing zfs to use 4k sectors (ashift= 12) even if in your case there is good chance that 512b sectors (ashift= 9) would be the default setting. Again, for later upgrade as you really, really don't want to mix ashift in a pool.

Edit: mixed vdev in a pool is possible, but not recommended (thanks @bahamat) and warning about "ashift".