system call -> vfs -> ext3 or others -> block device or network
File is basic unit and consistents of attributes(acl, also known as metadata) and user data
Use tree to management
file_system_type, ext4 or xfs, how to build super block
one block device has only one super block instance
inode consistents of metadata
dentry is the location of file tree, one inode can has multiple dentry(such as hard link)
mount instance is the mount location, can mount more than one time
the <mount, dentry> let each file system has its own view
hash string file name to inode and file struct
async write, wirte to page addir buffer, let BDI thread to write
sync write, should set sync when open file, wait for sync
透写（o_direct）, more strict, write old then new and clean cache