**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 :)

assert(!tmp_part_prefix.empty());
    String tmp_dst_part_name = tmp_part_prefix + dst_part_name;
    auto temporary_directory_lock = getTemporaryPartDirectoryHolder(tmp_dst_part_name);
    /
    auto reservation = src_part->data_part_storage->reserve(src_part->getBytesOnDisk());
    auto src_part_storage = src_part->data_part_storage;
    /
    if (auto src_part_in_memory = asInMemoryPart(src_part))
    {
        auto flushed_part_path = src_part_in_memory->getRelativePathForPrefix(tmp_part_prefix);
        src_part_storage = src_part_in_memory->flushToDisk(flushed_part_path, metadata_snapshot);
    }
    String with_copy;
    if (copy_instead_of_hardlink)
        with_copy = " (copying data)";
    LOG_DEBUG(log, "Cloning part {} to {}{}",
              src_part_storage->getFullPath(),
              std::string(fs::path(src_part_storage->getFullRootPath()) / tmp_dst_part_name),
              with_copy);
    auto dst_part_storage = src_part_storage->freeze(relative_data_path, tmp_dst_part_name, / false, {}, / copy_instead_of_hardlink);
    auto dst_data_part = createPart(dst_part_name, dst_part_info, dst_part_storage);
    if (!copy_instead_of_hardlink && hardlinked_files)
    {
        hardlinked_files->source_part_name = src_part->name;
        hardlinked_files->source_table_shared_id = src_part->storage.getTableSharedID();
        for (auto it = src_part->data_part_storage->iterate(); it->isValid(); it->next())
        {
            if (it->name() != IMergeTreeDataPart::DELETE_ON_DESTROY_MARKER_FILE_NAME && it->name() != IMergeTreeDataPart::TXN_VERSION_METADATA_FILE_NAME)
                hardlinked_files->hardlinks_from_source_part.insert(it->name());
        }