How is data written to a pool with multiple VDevs

[Artem Belevich]

On Tue, Aug 31, 2010 at 5:18 PM, Danny Carroll <fbsd at dannysplace.net> wrote:
...
> My question is:  Since the data will initially be on only one of the
> raidz vdevs, when I add the second one, will ZFS re-organise the data on
> the whole pool to be efficient or will I end up with only new data being
> striped.

No, I believe ZFS will not reorganize the data that's already written.
However, ZFS will make an attempt to spread the data across both vdev
once both are available.

> If that is the case then I will try and find a way to build the new
> array fully on a temporary machine first before copying the data.

I guess once you've added second vdev and provided that you have
enough space left, you can try copying your dataset once again. New
copy will be spread across both vdevs.

--Artem