**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 :)

{
    temporary_parts.add(part_dir_name);
    return [this, part_dir_name]() { temporary_parts.remove(part_dir_name); };
}
MergeTreeData::DataPartsVector MergeTreeData::grabOldParts(bool force)
{
    DataPartsVector res;
    
    std::unique_lock lock(grab_old_parts_mutex, std::defer_lock);
    if (!lock.try_lock())
        return res;
    bool need_remove_parts_in_order = supportsReplication() && getSettings()->allow_remote_fs_zero_copy_replication;
    if (need_remove_parts_in_order)
    {
        bool has_zero_copy_disk = false;
        for (const auto & disk : getDisks())
        {
            if (disk->supportZeroCopyReplication())
            {
                has_zero_copy_disk = true;