**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 :)

{
    part->assertState({DataPartState::Temporary});
    if (DataPartPtr existing_part_in_partition = getAnyPartInPartition(part->info.partition_id, lock))
    {
        if (part->partition.value != existing_part_in_partition->partition.value)
            throw Exception(
                "Partition value mismatch between two parts with the same partition ID. Existing part: "
                + existing_part_in_partition->name + ", newly added part: " + part->name,
                ErrorCodes::CORRUPTED_DATA);
    }
    if (auto it_duplicate = data_parts_by_info.find(part->info); it_duplicate != data_parts_by_info.end())
    {
        String message = "Part " + (*it_duplicate)->getNameWithState() + " already exists";
        if ((*it_duplicate)->checkState({DataPartState::Outdated, DataPartState::Deleting}))
            throw Exception(message + ", but it will be deleted soon", ErrorCodes::PART_IS_TEMPORARILY_LOCKED);
        throw Exception(message, ErrorCodes::DUPLICATE_DATA_PART);
    }
}
void MergeTreeData::preparePartForCommit(MutableDataPartPtr & part, Transaction & out_transaction, DataPartStorageBuilderPtr builder)