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. - 

}
    }
    setDataVolume(0, 0, 0);
    LOG_TRACE(log, "dropAllData: done.");
}
void MergeTreeData::dropIfEmpty()
{
    LOG_TRACE(log, "dropIfEmpty");
    auto lock = lockParts();
    if (!data_parts_by_info.empty())
        return;
    try
    {
        for (const auto & [path, disk] : getRelativeDataPathsWithDisks())
        {
            
            disk->removeFileIfExists(fs::path(path) / MergeTreeData::FORMAT_VERSION_FILE_NAME);
            disk->removeDirectory(fs::path(path) / MergeTreeData::DETACHED_DIR_NAME);
            disk->removeDirectory(path);
        }
    }
    catch (...)
    {
        
        tryLogCurrentException(__PRETTY_FUNCTION__);
    }
}
namespace
{
bool isSafeForKeyConversion(const IDataType * from, const IDataType * to)