**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 :)

{
    out = disk->writeFile(path, DBMS_DEFAULT_BUFFER_SIZE, WriteMode::Append);
    
    
    block_out = std::make_unique<NativeWriter>(*out, 0, Block{});
    min_block_number = std::numeric_limits<Int64>::max();
    max_block_number = -1;
    bytes_at_last_sync = 0;
}
void MergeTreeWriteAheadLog::addPart(DataPartInMemoryPtr & part)
{
    std::unique_lock lock(write_mutex);
    auto part_info = MergeTreePartInfo::fromPartName(part->name, storage.format_version);
    min_block_number = std::min(min_block_number, part_info.min_block);
    max_block_number = std::max(max_block_number, part_info.max_block);
    writeIntBinary(WAL_VERSION, *out);
    ActionMetadata metadata{};
    metadata.part_uuid = part->uuid;
    metadata.write(*out);
    writeIntBinary(static_cast<UInt8>(ActionType::ADD_PART), *out);
    writeStringBinary(part->name, *out);
    block_out->write(part->block);
    block_out->flush();
    sync(lock);
    auto max_wal_bytes = storage.getSettings()->write_ahead_log_max_bytes;
    if (out->count() > max_wal_bytes)
        rotate(lock);
}
void MergeTreeWriteAheadLog::dropPart(const String & part_name)