There's two types of "data" in the pool: the actual data of whatever it is you're storing, and the metadata, which is all the tables, properties, indexes and other "stuff" that defines the pool structure, the datasets, and the pointers that tell ZFS where on disk to find the actual data.

Normally this is all mixed in together on the regular pool vdevs (mirror, raidz, etc). If you add a special vdev to your pool, then ZFS will prefer to store the metadata there, and send the data proper to the regular vdevs. The main reason for doing this is if you have "slow" data vdevs; adding a special vdev of a SSD mirror can speed up access times, as ZFS can look to the SSDs to know where on the data vdevs to find its data and go directly there, rather than loading the metadata off the slow vdevs and then needing another access to get the real data.

There's another possible advantage: ZFS can store "small" files on the special vdev, leaving larger ones for the regular data vdevs.

One important thing to remember is that special vdevs are a proper part of the pool, not an add-on - just like the regular data vdevs, if the special vdev fails, the pool is lost. An SSD mirror is typical for this vdev.

This is only a rough explanation, see also:

• zpoolconcepts(7)
• Level1Techs writeup

zpool list -v -H -P

-v verbose
-P show full paths, not just the last component
-H script mode - no headings, fields separated by tab character

That will get you a lot closer.

After you setup your conf file you need to do a udevadm trigger for your new devices to show up it will make a diretory under /dev/disk called by-vdev that will have the alias names for all your disk in it.

Don Brady presented on this work at the Second (edit: First?) annual ZFS user conference in Norwalk, CT sponsored by Datto. It was required by the DRAID work Intel is working on for Fermilab and perhaps other Nuclear tech labs across the U.S. very interesting work, I haven't had a need to implement it yet but am interested in hearing about deployments. One of the ZFS mailing lists had a complaint from a user that added a special Allocation VDEV, played around with it, and then found they couldn't delete it easily like a SLOG or l2arc device. N.B. For now, it is permanent once added , pool must be copied and destroyed and rewritten if you change your mind.

here is the link to the presentation document : https://zfs.datto.com/2017_slides/brady.pdf