**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 :)

{
    if (getSettings()->in_memory_parts_enable_wal)
        return;
    auto metadata_snapshot = getInMemoryMetadataPtr();
    DataPartsVector parts = getDataPartsVectorForInternalUsage();
    for (const auto & part : parts)
    {
        if (auto part_in_memory = asInMemoryPart(part))
        {
            part_in_memory->flushToDisk(part_in_memory->data_part_storage->getPartDirectory(), metadata_snapshot);
        }
    }
}
size_t MergeTreeData::clearOldPartsFromFilesystem(bool force)
{
    DataPartsVector parts_to_remove = grabOldParts(force);
    clearPartsFromFilesystem(parts_to_remove);
    removePartsFinally(parts_to_remove);
    
    
    if (!parts_to_remove.empty())
        getContext()->dropMMappedFileCache();
    return parts_to_remove.size();
}
void MergeTreeData::clearPartsFromFilesystem(const DataPartsVector & parts, bool throw_on_error, NameSet * parts_failed_to_delete)