Looking at logs, it seems that number of requests to figure out what could be deleted and what not is calculated as number of shards x number of snapshots. In our case with daily indices for a year of data and 37 snapshots this is roughly 75000 requests. In another month it would be 150k.

If you start making daily shapshots with 2000 shards and 5 new shards every day, at the end of the year you'll need 214634600 (214 million!) requests to remove single snapshot.

This takes a lot of time to finish. Maybe there could be some optimization with denormalization to speed things up a bit.

I realize that removing a snapshot is not that frequent operation :)