Expand a zpool with a single supporting device on linux

Initial status:

Because I have had trouble with the way ZFS on Linux (ZOL) handles failing drives, I have decided to create a software
RAID set of my drives before creating my zpool.

This means that my zpool is created with a single device (/dev/md0). If I remember correctly, it was created like this:

zpool create pool0 md0

Expansion:

By default pool’s are not set to autoexpand so it is needed to set the autoexpand to on:

zpool set autoexpand=on pool0

Before I could get the pool expanded I needed to grow my RAID set. It is done by first adding a device to the existing
RAID set and then “growing” it with the new amount of devices.

mdadm /dev/md0 --add /dev/sdd
mdadm --grow /dev/md0 --raid-devices=4

After the RAID set have reshaped, the extra space is available for use. According to all information I could find,
it should be possible to expand the pool by either exporting and importing it

zpool export pool0
zpool import pool0

or by expanding the online pool

zpool online -e pool0 md0

I trid both several times and in different order, but to no avail. My pool was fixed at its original size.

Reboot:

Finally I decided to do a reboot to see if that would change anything. Remember to save the new mdadm configuration before
rebooting.

# Remove existing array configuration from mdadm.conf file
grep -v ARRAY /etc/mdadm/mdadm.conf > /tmp/mdadm.conf
# Add new array configuration to mdadm.conf
cat /tmp/mdadm.conf > /etc/mdadm/mdadm.conf
mdadm --examine --scan >> /etc/mdadm/mdadm.conf

After the reboot the pool was still the original size, but now it actually worked to do an online expansion.

zpool online -e pool0 md0

And that solved the problem. Unfortunately I do not know why I could not perform the online expansion without a reboot.

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