is snapshot garbage collection incremental or atomic? e.g. if i have a bunch of snapshots over a 24hr period and i bump the GC window down to 5min and suddenly there's a small hill's worth of snapshots to GC, am I going to make the GC process to fall over because it's going to try and do all of them at the same time and choke? or will it incrementally make progress even if an individual GC run times out?