aledavsanort - Tuesday, January 23, 2018 10:22 AM
as stated this is by design on the differential backups. The only real pressure is IO and if you are backing up to an appliance whose sole purpose is for backups only then that pressure is mitigated against any IO pressure on the database(s) data and log files.
Just as a note if you have a custom backup solution to take care of your entire enterprise of sql servers, you will note that when you issue a differential backup command it errors out on the secondary's. There is a quick check you can do to determine if a database is a secondary replica or not.
lookup the use of fn_hadr_backup_is_preferred_replica. I essentially check for the existence of the function and then set a bit flag to 0 if it does NOT exist so I know to bypass that database on backups. Thus all of my backups are against the primary, of course you could create logic to work everything but a differential against a secondary but I found it easier to keep the log chain as close to the primary as I could.