**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 :)

{
    DetachedPartsInfo res;
    for (const auto & disk : getDisks())
    {
        String detached_path = fs::path(relative_data_path) / MergeTreeData::DETACHED_DIR_NAME;
        /
        if (disk->exists(detached_path))
        {
            for (auto it = disk->iterateDirectory(detached_path); it->isValid(); it->next())
            {
                res.push_back(DetachedPartInfo::parseDetachedPartName(disk, it->name(), format_version));
            }
        }
    }
    return res;
}
void MergeTreeData::validateDetachedPartName(const String & name)
{
    if (name.find('/') != std::string::npos || name == "." || name == "..")
        throw DB::Exception("Invalid part name '" + name + "'", ErrorCodes::INCORRECT_FILE_NAME);
    if (startsWith(name, "attaching_") || startsWith(name, "deleting_"))
        throw DB::Exception("Cannot drop part " + name + ": "
                            "most likely it is used by another DROP or ATTACH query.",
                            ErrorCodes::BAD_DATA_PART_NAME);
}
void MergeTreeData::dropDetached(const ASTPtr & partition, bool part, ContextPtr local_context)