su _postgresql -c "cd /var/postgresql &&
    pg_upgrade --link -b /usr/local/bin/postgresql-9.6/ -B /usr/local/bin
    -U postgres -d /var/postgresql/data-9.6/ -D /var/postgresql/data"

CREATE TABLESPACE "mydb" OWNER "mydb" LOCATION '/path/to/postgresql/mydb'; psql:pg_upgrade_dump_globals.sql:87: ERROR:  directory "/path/to/postgresql/mydb/PG_10_201707211" already in use as a tablespace

[root@host:~]$ls /path/to/postgresql/dnslog/
PG_10_201707211   PG_9.6_201608131