1. ECS uses its own proprietary database to store the index 2. The index size is unlimited.  ECS shares all its disks for storing both data and index, so there's no limitations where only some drives are storing the index. 3. The internal limit is 2MB per object, but due to the way the S3 protocol works, you will only get around 4kB of metadata per object before you reach the maximum size of HTTP headers.
1. What is used to to provide the ECS Index?   2. How large can the Index be on ECS 2.2?   3. Is there a limit on the amount of metadata per object?  If so, what's the maximum? 
ECS Index Questions