After a kernel update, I noticed my backups in backintime took very long time. The backups also didn’t increment, every single backup used the same space -> several GB. Incremental backups should only take some MB, the amount of data that have been changed.
First I thought this was a bug in BIT, then zfs-fuse, but the error was in NFS4. When logged in to the server, I could create hard links. NFS4 could not do hard links. If I changed mount options to NFS3, hard links worked. But I don’t want to use NFS3 since NFS4 got username/id linking, so you don’t need the same userids on all of your systems. Seems like this is also why hard links didn’t work. Not sure if the error is because the new kernel does not enable idmapd by default, or if /etc/default/nfs-common was changed in the update. Anyhow, be sure that NEED_IDMAPD equals to yes in /etc/default/nfs-common on both server and clients.