Why isn't my ZFS pool expanding using ZFS on Linux?

I'm running ZFS on Ubuntu 16.04 and after much trial and error, this is is what worked for expanding the disk and pool size without rebooting. My system is hosted in the cloud at Profitbricks and uses libvirt (not SCSI) drives.

Get pool and device details:

# zpool status -v
   ...
    NAME        STATE     READ WRITE CKSUM
    pool        ONLINE       0     0     0
      vdb       ONLINE       0     0     0

# zpool list
NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
pool  39.8G  27.1G  12.7G         -    49%    68%  1.00x  ONLINE  -

Activate autoexpand:

# zpool set autoexpand=on pool

Now login to Profitbricks control panel and increase disk size from 40GB to 50GB.

Notify system of disk size change and expand pool:

# partprobe
Warning: Not all of the space available to /dev/vdb appears to be used,
you can fix the GPT to use all of the space (an extra 10485760 blocks) or 
continue with the current setting?

# zpool online -e pool vdb

# partprobe

# zpool list
NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
pool  49.8G  27.1G  21.7G         -    40%    55%  1.00x  ONLINE  -

I'm not sure why, but it is sometimes necessary to run partprobe and/or zpool online -e pool vdb twice in order to make the changes effective.

I read on the freebsd forums a post which suggested to use zpool online -e <pool> <vdev> (without needing to offline the vdev first)

This ultimately was the solution, but it required that ZFS autoexpand be disabled first:

[root@timestandstill ~]# zpool list
NAME          SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
dfbackup      214G   207G  7.49G    96%  1.00x  ONLINE  -
[root@timestandstill ~]# zpool get autoexpand
NAME         PROPERTY    VALUE   SOURCE
dfbackup     autoexpand  on      local
[root@timestandstill ~]# zpool set autoexpand=off dfbackup
[root@timestandstill ~]# zpool online -e dfbackup /dev/disk/by-id/virtio-sbs-XLPH83
[root@timestandstill ~]# zpool list
NAME          SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
dfbackup      249G   207G  42.5G    82%  1.00x  ONLINE  -

Using zpool set autoexpand=off followed by zpool online -e was required to get the zpool to expand for me, using ZFS on linux (in kernel, not using FUSE)