**Describe the bug**

Since we upgraded to 20.10.3.30, one of our clickhouse cluster takes much more time to start. (Up to 50 minutes, and before it took ~3 minutes)
The log file is filled with
>  <Debug> DiskLocal: Reserving 1.00 MiB on disk `default`, having unreserved 16.90 TiB

Additionally, the memory consumption increases, linearly, a lot (up to arround 230 G for us) and then drop back to normal values once clickhouse finished starting up. During that loading phase, a single CPU core is used at 100%.
If that memory is not available for the machine, clickhouse gets OOMed by the kernel.


**How to reproduce**
* Issue appeared after we upgraded to ClickHouse server version 20.10.3.30

* How could I find wich of our tables may be responsible for that ?


**Expected behavior**
A fast startup :)

{
    LOG_TRACE(log, "dropIfEmpty");
    auto lock = lockParts();
    if (!data_parts_by_info.empty())
        return;
    try
    {
        for (const auto & disk : getDisks())
        {
            if (disk->isBroken())
                continue;
            
            disk->removeFileIfExists(fs::path(relative_data_path) / MergeTreeData::FORMAT_VERSION_FILE_NAME);
            disk->removeDirectory(fs::path(relative_data_path) / MergeTreeData::DETACHED_DIR_NAME);
            disk->removeDirectory(relative_data_path);
        }
    }
    catch (...)
    {
        
        tryLogCurrentException(__PRETTY_FUNCTION__);
    }
}
namespace
{
bool isSafeForKeyConversion(const IDataType * from, const IDataType * to)