The difference between 'zpool add' & 'zpool attach'

Both sub-commands add & attach are used to configure additional storage for the zpool but are different in the way they function.

'zpool add' simply adds a vdev to a zpool.

In the below example I created a zpool called tpool & then added a vdev to it:

root@1z0822:~# zpool list tpool
NAME   SIZE  ALLOC  FREE  CAP  DEDUP  HEALTH  ALTROOT
tpool  195M   153K  195M   0%  1.00x  ONLINE  -

root@1z0822:~# zpool add tpool /root/disk2

root@1z0822:~# zpool list tpool
NAME   SIZE  ALLOC  FREE  CAP  DEDUP  HEALTH  ALTROOT
tpool  390M   122K  390M   0%  1.00x  ONLINE  -
root@1z0822:~#

The result was a simple concat operation.

In the second example I attached a vdev to the zpool:

root@1z0822:~# zpool list tpool

NAME   SIZE  ALLOC  FREE  CAP  DEDUP  HEALTH  ALTROOT

tpool  195M   128K  195M   0%  1.00x  ONLINE  -

root@1z0822:~# zpool attach tpool /root/disk1 /root/disk2

root@1z0822:~# zpool list tpool

NAME   SIZE  ALLOC  FREE  CAP  DEDUP  HEALTH  ALTROOT

tpool  195M   126K  195M   0%  1.00x  ONLINE  -

root@1z0822:~#

root@1z0822:~# zpool status tpool

  pool: tpool

 state: ONLINE

  scan: resilvered 88K in 0h0m with 0 errors on Sat Apr 23 21:30:32 2016

config:

        NAME             STATE     READ WRITE CKSUM

        tpool            ONLINE       0     0     0

          mirror-0       ONLINE       0     0     0

            /root/disk1  ONLINE       0     0     0

            /root/disk2  ONLINE       0     0     0

errors: No known data errors

root@1z0822:~#

A zpool attach operation results in the creation of a mirror.

Hope this helps clarify the difference between zpool add/attach.