**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 :)

, ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
    }
}
}
void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, ContextPtr local_context) const
{
    
    StorageInMemoryMetadata new_metadata = getInMemoryMetadata();
    StorageInMemoryMetadata old_metadata = getInMemoryMetadata();
    const auto & settings = local_context->getSettingsRef();
    const auto & settings_from_storage = getSettings();
    if (!settings.allow_non_metadata_alters)
    {
        auto mutation_commands = commands.getMutationCommands(new_metadata, settings.materialize_ttl_after_modify, getContext());
        if (!mutation_commands.empty())
            throw Exception(ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN, "The following alter commands: '{}' will modify data on disk, but setting `allow_non_metadata_alters` is disabled", queryToString(mutation_commands.ast()));
    }
    commands.apply(new_metadata, getContext());
    
    NameSet columns_alter_type_forbidden;
    
    
    NameSet columns_alter_type_metadata_only;
    
    NameSet columns_alter_type_check_safe_for_partition;
    if (old_metadata.hasPartitionKey())
    {
        
        auto partition_key_expr = old_metadata.getPartitionKey().expression;
        for (const auto & action : partition_key_expr->getActions())