As I’m running McAfee and not Sophos I’ll see what I can do to answer your questions.
With McAfee we successfully run two or more primary AV scanners against the same Filer.
I was wondering do your Sophos scanners have unique IP addresses and host names?
ONTAP will load balance between primary scanners.
ONTAP will load balance between secondary scanners.
Due to the way primary and secondary scanners are architected within ONTAP there is no load balancing between primary and secondary scanners.
I would suggest opening a support case if you have not done so already to determine why to are unable to have only one concurrent Sophos scanner.
By default all scanners registered to a filer will be primary unless configured to be a secondary scanner using the ‘vscan scanners secondary_scanners’ command (answers question 3).
ONTAP implements a basic round robin approach to load balancing that is built into its v-policy api (answers question 4).
For each file request that requires an AV scan ONTAP will send the next file to the subsequent scanner in the scanning pool.
Use the vscan command with a running workload and you will see the scan requests equally divided between the number of primary AV scanners registered with the Filer.
The following example shows that with two primary scanners that the requests are evenly balanced (answers question 1)
Toaster> vscan
Virus scanning is enabled.
Virus scanners(IP and Name) P/S Connect time (dd:hh:mm) Reqs Fails Curr. Reqs.