> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

{
    if (!max_size_to_drop || size <= max_size_to_drop)
        return;
    fs::path force_file(getFlagsPath() + "force_drop_table");
    bool force_file_exists = fs::exists(force_file);
    if (force_file_exists)
    {
        try
        {
            fs::remove(force_file);
            return;
        }
        catch (...)
        {
            
            tryLogCurrentException("Drop table check", "Can't remove force file to enable table or partition drop");
        }
    }
    String size_str = formatReadableSizeWithDecimalSuffix(size);
    String max_size_to_drop_str = formatReadableSizeWithDecimalSuffix(max_size_to_drop);
    throw Exception(ErrorCodes::TABLE_SIZE_EXCEEDS_MAX_DROP_SIZE_LIMIT,
                    "Table or Partition in {}.{} was not dropped.\nReason:\n"
                    "1. Size ({}) is greater than max_[table/partition]_size_to_drop ({})\n"
                    "2. File '{}' intended to force DROP {}\n"
                    "How to fix this:\n"