Currently, there is not a good way for counting orphaned documents in a sharded cluster.

The current approach is to run a

db.collection.find({ShardKey:{$gte: MinKey, $lte: MaxKey}},{ShardKey:1,_id:0}).itcount()

and compare this to the sum of shard counts individually.

• This query requires a full index scan, streaming the entire results to the shell. This can take significant time to complete on a large sharded cluster.
• On a busy system these numbers could be off significantly due to inserts and deletes interleaving during the count.
• Requires multiple queries to multiple hosts, leading to timing errors.

Since the logic already exists to cleanup orphans, a similar command (or parameter to the existing cleanupOrphaned) to count them would be useful for determining the potential impact of orphans on a given sharded cluster.