mypool
    mirror1
        disk1 (2TB)
        disk2 (2TB)
        combined
            disk3 (1TB)
            disk4 (1TB)

mypool
    mirror1
        disk1 (2TB)
        disk2 (2TB)

# mkdir zfs-test; cd zfs-test
# truncate -s 2G drive1; truncate -s 2G drive2
# truncate -s 1G drive3; truncate -s 1G drive4

# for N in $(jot 4); do mdconfig -u $N -t vnode -f drive$N; done
# mdconfig -lv
md1     vnode    2048M  /home/jim/zfs-test/drive1
md2     vnode    2048M  /home/jim/zfs-test/drive2
md3     vnode    1024M  /home/jim/zfs-test/drive3
md4     vnode    1024M  /home/jim/zfs-test/drive4

# zpool create mypool mirror md1 md2
# zpool status mypool
  pool: mypool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            md1     ONLINE       0     0     0
            md2     ONLINE       0     0     0

errors: No known data errors

# gstripe label -h combined md3 md4
# gstripe status
           Name  Status  Components
stripe/combined      UP  md3
                         md4
# zpool attach mypool md2 stripe/combined
cannot attach stripe/combined to md2: device is too small

# diskinfo -v md1 md2 stripe/combined
md1
        512             # sectorsize
        2147483648      # mediasize in bytes (2.0G)
        4194304         # mediasize in sectors
        0               # stripesize
        0               # stripeoffset
        MD-DEV5473951480393710199-INO24 # Disk ident.
        Yes             # TRIM/UNMAP support
        Unknown         # Rotation rate in RPM

md2
        512             # sectorsize
        2147483648      # mediasize in bytes (2.0G)
        4194304         # mediasize in sectors
        0               # stripesize
        0               # stripeoffset
        MD-DEV5473951480393710199-INO24 # Disk ident.
        Yes             # TRIM/UNMAP support
        Unknown         # Rotation rate in RPM

stripe/combined
        512             # sectorsize
        2147352576      # mediasize in bytes (2.0G)
        4194048         # mediasize in sectors
        65536           # stripesize
        0               # stripeoffset
        No              # TRIM/UNMAP support
        Unknown         # Rotation rate in RPM

# gstripe destroy combined
# mdconfig -d -u3; mdconfig -d -u4
# mdconfig -lv
md1     vnode    2048M  /home/jim/zfs-test/drive1
md2     vnode    2048M  /home/jim/zfs-test/drive2

# truncate -s 1025M drive3
# truncate -s 1025M drive4
# mdconfig -u3 -t vnode -f drive3
# mdconfig -u4 -t vnode -f drive4
# gstripe label -h combined md3 md4
# zpool attach mypool md2 stripe/combined
# zpool status mypool
  pool: mypool
 state: ONLINE
  scan: resilvered 81.5K in 0 days 00:00:04 with 0 errors on Thu May 23 15:27:26 2019
config:

        NAME                 STATE     READ WRITE CKSUM
        mypool               ONLINE       0     0     0
          mirror-0           ONLINE       0     0     0
            md1              ONLINE       0     0     0
            md2              ONLINE       0     0     0
            stripe/combined  ONLINE       0     0     0

errors: No known data errors