TL;DR

As the nextcloud docs say... if you want to write to an external volume that location has to be writeable by the user/group www-data on the host system... so if that makes sense to you then this TIL probably isn't a ton of value.. if not however, read on :)

Case Study

You want to self-host your own cloud and use a smart file system for convenience... Nextcloud and ZFS are pretty common goto answers for each of those problems.

My home NAS is built on ZFS and among other things I have a zpool named tank and nested in there is a tank/nas dataset with several child zfs datasets under that.

I want to use nextcloud mainly for auto-uploading photos from my wife's and my phones for automatic backups. The issue is that the nextcloud application (I run in Docker) is fixed as the www-data user and so any volume/folder that you want nextcloud to write to needs to be permissioned such that www-data owns it... but I don't want www-data to own everything in my NAS... so what's a girl to do?

Solution

Well, one way to go is to just utilize docker volumes, write the data in the container to /var/www/html and let that be the place your data backsup to.

I still wanted nextcloud to automatically write right to my NAS so I created a nextcloud-upload directory inside of tank/nas/media/photos (photos cause that's all that gets automatically uploaded)

Then I chown -R www-data:www-data /tank/nas/media/photos/nextcloud-upload so that just that sub-folder is owned by www-data. Now everyone's happy!