operation sysytem file type: mdf&ldf
database file type: mdf&ndf
===primary data file
include startup infomation for the database catalog and points to the outher database files
can also contain obj&data, that means can not contain obj&data
当用户自己创建用户自定义文件组，并制定该文件组为默认文件组之后，所有的数据库对象将存在该文件组中
主数据文件保存在主文件组中。主文件组不一定是默认文件组，默认文件组也不一定是主文件组
===secondary data file[Max:32776]
obj&user data
***put each sdf on a different disk drive to boost performance
===log file
records all database modifications that each transaction make
create multiple log files per database to facilitate faster recovery
===file group: a logical structure = pfg&ufg
splitting database objects across several filegroups, you can
#take advantage of the differrentdisk subsystems
#allow SQL Server to perform paralled disk operations
#back up and restore files individually
***default filegroup
***read-only filegroup
===how to design table
one access-intensive table:create multiple second data file,
store the files on different disk drives,
and group these files in a filegroup or multiple filegroups
===how to design filegroup
create at least one user-defined filegroup to hold seconday data files and database objects.
Configure this filegroup as the default filegroup so that SQL Server will store all objects
you create in this filegroup
==how to design database files with RAID system
RAID10------Log Files
RAID5-------Data Files
Configuration Database Files and Filegroups
