**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 :)

MutableDataPartPtr & part,
    Transaction & out_transaction,
    DataPartStorageBuilderPtr builder,
    DataPartsLock & lock)
{
    DataPartsVector covered_parts;
    if (!renameTempPartAndReplaceImpl(part, out_transaction, lock, builder, &covered_parts))
        return false;
    if (!covered_parts.empty())
        throw Exception("Added part " + part->name + " covers " + toString(covered_parts.size())
            + " existing part(s) (including " + covered_parts[0]->name + ")", ErrorCodes::LOGICAL_ERROR);
    return true;
}
void MergeTreeData::checkPartCanBeAddedToTable(MutableDataPartPtr & part, DataPartsLock & lock) const