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. - 

DataPartsVector parts;
    if (moving_part)
    {
        auto part_info = MergeTreePartInfo::fromPartName(partition_id, format_version);
        parts.push_back(getActiveContainingPart(part_info));
        if (!parts.back() || parts.back()->name != part_info.getPartName())
            throw Exception("Part " + partition_id + " is not exists or not active", ErrorCodes::NO_SUCH_DATA_PART);
    }
    else
        parts = getDataPartsVectorInPartition(MergeTreeDataPartState::Committed, partition_id);
    auto disk = getStoragePolicy()->getDiskByName(name);
    if (!disk)
        throw Exception("Disk " + name + " does not exists on policy " + getStoragePolicy()->getName(), ErrorCodes::UNKNOWN_DISK);
    parts.erase(std::remove_if(parts.begin(), parts.end(), [&](auto part_ptr)
        {
            return part_ptr->volume->getDisk()->getName() == disk->getName();
        }), parts.end());
    if (parts.empty())
    {
        String no_parts_to_move_message;
        if (moving_part)
            no_parts_to_move_message = "Part '" + partition_id + "' is already on disk '" + disk->getName() + "'";
        else
            no_parts_to_move_message = "All parts of partition '" + partition_id + "' are already on disk '" + disk->getName() + "'";
        throw Exception(no_parts_to_move_message, ErrorCodes::UNKNOWN_DISK);
    }
    if (!movePartsToSpace(parts, std::static_pointer_cast<Space>(disk)))
        throw Exception("Cannot move parts because moves are manually disabled", ErrorCodes::ABORTED);
}
void MergeTreeData::movePartitionToVolume(const ASTPtr & partition, const String & name, bool moving_part, ContextPtr local_context)