> (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 (!settings.max_read_buffer_size)
    {
        throw Exception(ErrorCodes::INVALID_SETTING_VALUE,
            "Invalid value '{}' for max_read_buffer_size", settings.max_read_buffer_size);
    }
    res.local_fs_buffer_size = settings.max_read_buffer_size;
    res.remote_fs_buffer_size = settings.max_read_buffer_size;
    res.direct_io_threshold = settings.min_bytes_to_use_direct_io;
    res.mmap_threshold = settings.min_bytes_to_use_mmap_io;
    res.priority = settings.read_priority;
    res.remote_throttler = getRemoteReadThrottler();
    res.http_max_tries = settings.http_max_tries;
    res.http_retry_initial_backoff_ms = settings.http_retry_initial_backoff_ms;
    res.http_retry_max_backoff_ms = settings.http_retry_max_backoff_ms;
    res.http_skip_not_found_url_for_globs = settings.http_skip_not_found_url_for_globs;
    res.mmap_cache = getMMappedFileCache().get();
    return res;
}
WriteSettings Context::getWriteSettings() const
{
    WriteSettings res;
    res.enable_filesystem_cache_on_write_operations = settings.enable_filesystem_cache_on_write_operations;
    res.enable_filesystem_cache_log = settings.enable_filesystem_cache_log;
    res.throw_on_error_from_cache = settings.throw_on_error_from_cache_on_write_operations;
    res.s3_allow_parallel_part_upload = settings.s3_allow_parallel_part_upload;
    res.remote_throttler = getRemoteWriteThrottler();
    return res;
}
}