**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 :)

{
    DataPartsVector res;
    if (const auto * txn = local_context->getCurrentTransaction().get())
    {
        res = getDataPartsVectorForInternalUsage({DataPartState::Active, DataPartState::Outdated});
        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
    }
    else
    {
        res = getDataPartsVectorForInternalUsage();
    }
    return res;
}
DataPartsVector MergeTreeData::getVisibleDataPartsVectorUnlocked(ContextPtr local_context, const DataPartsLock & lock) const