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

In order to view the current value of a specific setting like ashift, you will need to use the zdb command instead of the zpool command.

Running zdb on its own with no arguments will give you a view of any pools found on the system, and their vdevs, and disks within the vdevs.

root@pve1:/home/tim# zdb
pm1:
    version: 5000
    name: 'pm1'
    state: 0
    txg: 801772
    pool_guid: 13783858310243843123
    errata: 0
    hostid: 2831164162
    hostname: 'pve1'
    vdev_children: 1
    vdev_tree:
        type: 'root'
        id: 0
        guid: 13783858310243843123
        children[0]:
            type: 'raidz'
            id: 0
            guid: 13677153442601001142
            nparity: 2
            metaslab_array: 34
            metaslab_shift: 33
            ashift: 9
            asize: 1600296845312
            is_log: 0
            create_txg: 4
            children[0]:
                type: 'disk'
                id: 0
                guid: 4356695485691064080
                path: '/dev/disk/by-id/ata-DENRSTE251M45-0400.C_A181B011241000542-part1'
                whole_disk: 1
                not_present: 1
                DTL: 64
                create_txg: 4
            children[1]:
                type: 'disk'
                id: 1
                guid: 14648277375932894482
                path: '/dev/disk/by-id/ata-DENRSTE251M45-0400.C_A181B011241000521-part1'
                whole_disk: 1
                DTL: 82
                create_txg: 4
            children[2]:
                type: 'disk'
                id: 2
                guid: 11362800770521042303
                path: '/dev/disk/by-id/ata-DENRSTE251M45-0400.C_A181B011241000080-part1'
                whole_disk: 1
                DTL: 59
                create_txg: 4
            children[3]:
                type: 'disk'
                id: 3
                guid: 10494331395233532833
                path: '/dev/disk/by-id/ata-DENRSTE251M45-0400.C_A181B011241000517-part1'
                whole_disk: 1
                DTL: 58
                create_txg: 4
    features_for_read:
        com.delphix:hole_birth
        com.delphix:embedded_data

or, for just ashift with some context:

root@pve1:/home/tim#  sudo zdb | egrep 'ashift|vdev|type' | grep -v disk
    vdev_children: 1
    vdev_tree:
        type: 'root'
            type: 'raidz'
            ashift: 9

Here is an old blog post about zdb that is still very informative about the origins and intent, and the information that comes out of zdb. A quick google also reveals many posts that may be more specifically relevant to ZFS on Linux.