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. - 

{
        const auto settings = getSettings();
        return settings->index_granularity_bytes != 0 &&
            (settings->enable_mixed_granularity_parts || !has_non_adaptive_index_granularity_parts);
    }
    
    
    
    MergeTreeSettingsPtr getSettings() const
    {
        return storage_settings.get();
    }
    String getRelativeDataPath() const { return relative_data_path; }
    
    String getFullPathOnDisk(const DiskPtr & disk) const;
    
    
    DiskPtr getDiskForPart(const String & part_name, const String & additional_path = "") const;
    
    
    std::optional<String> getFullRelativePathForPart(const String & part_name, const String & additional_path = "") const;
    bool storesDataOnDisk() const override { return true; }