**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 :)

{
    std::unique_lock lock(write_mutex);
    writeIntBinary(WAL_VERSION, *out);
    ActionMetadata metadata{};
    metadata.write(*out);
    writeIntBinary(static_cast<UInt8>(ActionType::DROP_PART), *out);
    writeStringBinary(part_name, *out);
    out->next();
    sync(lock);
}
void MergeTreeWriteAheadLog::rotate(const std::unique_lock<std::mutex> &)
{
    String new_name = String(WAL_FILE_NAME) + "_"
        + toString(min_block_number) + "_"
        + toString(max_block_number) + WAL_FILE_EXTENSION;
    
    out->finalize();
    disk->replaceFile(path, storage.getRelativeDataPath() + new_name);
    init();
}
MergeTreeData::MutableDataPartsVector MergeTreeWriteAheadLog::restore(const StorageMetadataPtr & metadata_snapshot, ContextPtr context)