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. - 

}
    }
    if (!has_been_already_added)
    {
        const auto storage_settings_ptr = getSettings();
        String part_path = fs::path(replica_path) / "parts" / part_name;
        
        
        if (storage_settings_ptr->use_minimalistic_part_header_in_zookeeper)
        {
            ops.emplace_back(zkutil::makeCreateRequest(
                part_path, local_part_header.toString(), zkutil::CreateMode::Persistent));
        }
        else
        {
            ops.emplace_back(zkutil::makeCreateRequest(
                part_path, "", zkutil::CreateMode::Persistent));
            ops.emplace_back(zkutil::makeCreateRequest(
                fs::path(part_path) / "columns", part->getColumns().toString(), zkutil::CreateMode::Persistent));
            ops.emplace_back(zkutil::makeCreateRequest(
                fs::path(part_path) / "checksums", getChecksumsForZooKeeper(part->checksums), zkutil::CreateMode::Persistent));
        }
    }
    else
    {
        LOG_WARNING(log, "checkPartAndAddToZooKeeper: node {} already exists. Will not commit any nodes.",
                    (fs::path(replica_path) / "parts" / part_name).string());
    }
}
MergeTreeData::DataPartsVector StorageReplicatedMergeTree::checkPartChecksumsAndCommit(Transaction & transaction,