> (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.

}
            }
            throw Exception(ErrorCodes::CANNOT_PARSE_BACKUP_SETTINGS, "Cannot parse creation mode from {}", field);
        }
        explicit operator Field() const
        {
            switch (value)
            {
                case RestoreAccessCreationMode::kCreate: return Field{true};
                case RestoreAccessCreationMode::kCreateIfNotExists: return Field{"if-not-exists"};
                case RestoreAccessCreationMode::kReplace: return Field{"replace"};
            }
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected value of enum RestoreAccessCreationMode: {}", static_cast<int>(value));
        }
    };
    using SettingFieldRestoreUDFCreationMode = SettingFieldRestoreAccessCreationMode;
}
#define LIST_OF_RESTORE_SETTINGS(M) \
    M(String, id) \
    M(String, password) \
    M(Bool, structure_only) \
    M(RestoreTableCreationMode, create_table) \
    M(RestoreDatabaseCreationMode, create_database) \
    M(Bool, allow_different_table_def) \
    M(Bool, allow_different_database_def) \
    M(Bool, async) \
    M(UInt64, shard_num) \
    M(UInt64, replica_num) \
    M(UInt64, shard_num_in_backup) \
    M(UInt64, replica_num_in_backup) \