Two and a half questions

1. Is it really necessary to run rsync with the -av command to also store user and group permissions?
2. if yes, how can I do that properly?
I am rsyncing /data and /home/meteohub to my NAS. I am running the following commands:
Code: Select all
su -c "rsync -av /data /mnt/nas_backup/_data" admin
su -c "rsync -av /home/meteohub /mnt/nas_backup/_config" admin
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rsync: chgrp "/mnt/nas_backup/_config/meteohub/histeval0" failed: Operation not permitted (1)
rsync: chgrp "/mnt/nas_backup/_config/meteohub/histeval1" failed: Operation not permitted (1)
rsync: chgrp "/mnt/nas_backup/_config/meteohub/histeval2" failed: Operation not permitted (1)
rsync: chgrp "/mnt/nas_backup/_config/meteohub/histeval3" failed: Operation not permitted (1)
rsync: chgrp "/mnt/nas_backup/_config/meteohub/lang.template" failed: Operation not permitted (1)