**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,
    DataPartsLock & lock,
    DataPartStorageBuilderPtr builder,
    DataPartsVector * out_covered_parts)
{
    LOG_TRACE(log, "Renaming temporary part {} to {}.", part->data_part_storage->getPartDirectory(), part->name);
    if (&out_transaction.data != this)
        throw Exception("MergeTreeData::Transaction for one table cannot be used with another. It is a bug.",
            ErrorCodes::LOGICAL_ERROR);
    if (part->hasLightweightDelete())
        has_lightweight_delete_parts.store(true);
    checkPartCanBeAddedToTable(part, lock);
    DataPartPtr covering_part;
    DataPartsVector covered_parts = getActivePartsToReplace(part->info, part->name, covering_part, lock);
    if (covering_part)
    {
        LOG_WARNING(log, "Tried to add obsolete part {} covered by {}", part->name, covering_part->getNameWithState());
        return false;
    }