Postgres not being able to create a tablespace
I just found a solution. I changed the permission of /mnt/DB/postgres to 700:

postgres@vostro /mnt/DB $ ls -l
total 4
drwx------ 3 postgres postgres 4096 May 25 17:46 postgres


Now it worked the creation of the ts

postgres@vostro /mnt/DB/postgres $ ls -l
total 4
drwx------ 2 postgres postgres 4096 May 25 17:46 PG_9.5_201510051




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