**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 :)

{
    Poco::JSON::Parser parser;
    auto json = parser.parse(buf).extract<Poco::JSON::Object::Ptr>();
    if (json->has(JSON_KEY_PART_UUID))
        part_uuid = parseFromString<UUID>(json->getValue<std::string>(JSON_KEY_PART_UUID));
}
void MergeTreeWriteAheadLog::ActionMetadata::read(ReadBuffer & meta_in)
{
    readIntBinary(min_compatible_version, meta_in);
    if (min_compatible_version > WAL_VERSION)
        throw Exception("WAL metadata version " + toString(min_compatible_version)
                        + " is not compatible with this ClickHouse version", ErrorCodes::UNKNOWN_FORMAT_VERSION);
    size_t metadata_size;
    readVarUInt(metadata_size, meta_in);
    if (metadata_size == 0)
        return;
    String buf(metadata_size, ' ');
    meta_in.readStrict(buf.data(), metadata_size);
    fromJSON(buf);
}
void MergeTreeWriteAheadLog::ActionMetadata::write(WriteBuffer & meta_out) const
{
    writeIntBinary(min_compatible_version, meta_out);
    String ser_meta = toJSON();
    writeVarUInt(static_cast<UInt32>(ser_meta.length()), meta_out);
    writeString(ser_meta, meta_out);
}
}