Re: ZFS keeping data seperate?
OK, I think I get it. You want to do performance isolation: you don't want workload on one disk (for example the root disk) to affect performance on another disk (for example the http disk). Although I find it unlikely that the root disk has any performance issues (since workload is usually minimal on root), and in most single-server configurations, the performance of a web server is unlikely to be limited by the underlying disk drive. But you are free to do it.
Now, can you combine that with mirroring, using just the last disk for mirroring? Absolutely. Here is one simple way to do it: On the last disk, create partitions, each the same size as one of the other disks. Then set up ZFS to use mirror pairs as volumes, for example (ada0 and ada4p1), (ada1 and ada4p2), and so on. But: In doing so, you defeat the purpose of performance isolation. Anytime you do a write to for example the root file system, ZFS will have to write (simulatenously) to both ada0 and ada4. And if you read from for example the http file system, ZFS will issue the read to either ada1 or ada4. So workload on one of these file systems will now compete for performance with the other, on the last disk. Personally, I don't trust disks, and I would always prefer a reliable system (using RAID, for example mirroring) over some performance, but this is your system.
Another option: Just use the last disk for backups. For example, create a giant file system on it, and regularly (every hour? every day?), rsync
the other file systems onto it. Now you have performance isolation. But you will regularly run performance-eating backups, and your backups are always slightly out of date, unlike mirrors.
Your other backup disk seems to be on another system (it has the same device name, ada0, as the boot disk of the first system). That means you can't use it as a mirror. In theory, you could export it using something like iSCSI or Infiniband, but network RAID is tricky (in particular from a performance point of view), and shouldn't be attempted using file systems that weren't designed for it.
Hope this helps.