**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 :)

{
    return NamesAndTypesList{
        NameAndTypePair("_part", std::make_shared<DataTypeString>()),
        NameAndTypePair("_part_index", std::make_shared<DataTypeUInt64>()),
        NameAndTypePair("_part_uuid", std::make_shared<DataTypeUUID>()),
        NameAndTypePair("_partition_id", std::make_shared<DataTypeString>()),
        NameAndTypePair("_partition_value", getPartitionValueType()),
        NameAndTypePair("_sample_factor", std::make_shared<DataTypeFloat64>()),
        NameAndTypePair("_part_offset", std::make_shared<DataTypeUInt64>()),
        LightweightDeleteDescription::FILTER_COLUMN,
    };
}
size_t MergeTreeData::getTotalMergesWithTTLInMergeList() const
{
    return getContext()->getMergeList().getMergesWithTTLCount();
}
void MergeTreeData::addPartContributionToDataVolume(const DataPartPtr & part)
{
    increaseDataVolume(part->getBytesOnDisk(), part->rows_count, 1);
}
void MergeTreeData::removePartContributionToDataVolume(const DataPartPtr & part)
{
    increaseDataVolume(-part->getBytesOnDisk(), -part->rows_count, -1);
}
void MergeTreeData::increaseDataVolume(ssize_t bytes, ssize_t rows, ssize_t parts)