**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 :)

{
    DataParts res;
    {
        auto lock = lockParts();
        for (auto state : affordable_states)
        {
            auto range = getDataPartsStateRange(state);
            res.insert(range.begin(), range.end());
        }
    }
    return res;
}
MergeTreeData::DataParts MergeTreeData::getDataPartsForInternalUsage() const
{
    return getDataParts({DataPartState::Active});
}
MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorForInternalUsage() const
{
    return getDataPartsVectorForInternalUsage({DataPartState::Active});
}
MergeTreeData::DataPartPtr MergeTreeData::getAnyPartInPartition(
    const String & partition_id, DataPartsLock & / const
{
    auto it = data_parts_by_state_and_info.lower_bound(DataPartStateAndPartitionID{DataPartState::Active, partition_id});
    if (it != data_parts_by_state_and_info.end() && (*it)->getState() == DataPartState::Active && (*it)->info.partition_id == partition_id)
        return *it;
    return nullptr;
}
MergeTreeData::Transaction::Transaction(MergeTreeData & data_, MergeTreeTransaction * txn_)