We have 3 clients backup to cloud, the backup size showed in NBU console is around 3TB, 3TB, 4TB. Using MS-Windows backup. Master + Media Server with Local cache directory attached S3 storage from Alibaba cloud.
For the 3 questions,
I guess, if the cache can't find related block, will it download the data from cloud?
4TB cache should be okay.
You really should be able to answer what type of storage you have configured.
If (and I say if) the storage is MSDP-Cloud or Cloud Catalyst, each data segment is finger printed. This finger print is compared to a cache on the local media server and the media server will only send the data segment to the cloud if it is new. There should be no reason for any (significant) data reads from the cloud during a normal backup operation.
It might be worth while opening a support case to have this investigated further (maybe there is something misconfigured that is casuing the excessive reads from the cloud).