Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

}
        else
        {
            
            part->is_temp = false;
            part->renameTo(fs::path("detached") / part_name, true);
        }
    }
    catch (const Exception & e)
    {
        
        
        if (e.code() == ErrorCodes::DIRECTORY_ALREADY_EXISTS)
            return false;
        throw;
    }
    catch (...)
    {
        if (!to_detached)
            write_part_log(ExecutionStatus::fromCurrentException());
        throw;
    }
    ProfileEvents::increment(ProfileEvents::ReplicatedPartFetches);
    if (part_to_clone)
        LOG_DEBUG(log, "Cloned part {} from {}{}", part_name, part_to_clone->name, to_detached ? " (to 'detached' directory)" : "");
    else
        LOG_DEBUG(log, "Fetched part {} from {}{}", part_name, source_replica_path, to_detached ? " (to 'detached' directory)" : "");
    return true;
}
bool StorageReplicatedMergeTree::fetchExistsPart(const String & part_name, const StorageMetadataPtr & metadata_snapshot,