Hello @xiwen 

This documentation should answer all of your questions.

https://documentation.commvault.com/11.24/expert/3319_system_requirements_for_virtual_server_agent_with_microsoft_azure_01.html

Please note the “Virtual Server Agent Proxy Requirements” section and all of its details. If there are multiple regions involved, you would want to deploy at least one proxy per region. If all one region, you can have one VSA proxy, however, depending on the load amount you may want to deploy more. You can use these guidelines Hardware Specifications for Virtual Server Agent (commvault.com)

Is it possible to store snaphots across subscriptions?  If you have 50 subscriptions we don’t want to setup a resource group for each subscription.  

@xiwen , hope all is well!

Have you checked out the docs on adding VM Groups from Azure?

https://documentation.commvault.com/11.24/essential/121771_adding_vm_group_for_azure.html

Take a look here and let me know if you have any questions.

I have five subscription in one Azure site, shall we use one VSA to manage all Azure VM(separate in five subscription) agentless backup job?

Depending on the regions for your subscriptions, you may want multiple Proxies/Agents, but if all subscriptions are in the same region, then one VSA should be fine.

Let me know if that helps!

For example:

I deploy VSA in subscription A, want to agentless protection for Azure VM in subscription B. Maybe VSA could call Azure App to create Azure VM snapshot(subscription , during backup copy process, how to mount Azure VM snapshot(subscription on VSA(subscription A)? any permission challenge?