Get index size on each shard?

Question

I have a compound index on a sharded cluster living on Mongo Atlas. The Atlas UI tells me the index size is 10.1GB. Similarly,

MongoDB Enterprise > db.myCollection.stats().indexSizes
{
    "_id_" : 14387392512,
    "my_index" : 10810015744
}

My cluster has two shards. Can I assume that there is actually 5GB of index on each shard, since each shard contains its own indexing? Is there a way to get the exact size of the index on each shard? If I add a third shard, will this index size be reduced to ~3.3GB per shard, assuming a balanced distribution?

I want to ensure that my index size + working set remains in RAM and find the right balance of vertical scaling (upgrading cluster tiers) vs. horizontal scaling (adding more shards) to accommodate this.

Answer 1

You are on correct path. Once collection is sharded the db.stats() will give stats of entire database and each shard documents/size/indexe size/total size and other things. We have created a report based on this to show daily shard distribution. This in python gives dictionary and easy to get values