Linux File systems The VFS

1
Linux File systems The VFS

• Advait Jain
• Priyanka Jindal

2
What is the VFS ?

• Component in the kernel that handles
  file-systems, directory and file access.
• Abstracts common tasks of many file-systems.
• Presents the user with a unified interface, via
  the file-related system calls (open, stat, chmod
  etc.).
• Filesystem-specific operations- vector them to
  the filesystem in charge of the file.

3
(No Transcript)
4
Mounting a device

• mount -t iso9660 -o ro /dev/cdrom /mnt/cdrom
• Steps involved
• Find the file system.(file_systems list)
• Find the VFS inode of the directory that is to be
  the new file system's mount point.
• Allocate a VFS superblock and call the file
  system specific read_super function.

5
List of all file systems
6
The currently mounted file systems
7
Opening a File

• Pathname is used by the VFS to search through the
  directory (pathname to device number, block
  number).
• Calls the lookup() method (file system specific)
  of the parent directory inode to look up the
  inode.
• Caches the directory entries during this
  traversal. (DCache)

8

• Check for permissions and if ok then proceed.
• Allocates a file structure with a pointer to its
  dentry and to file specific operations.
• File structure is placed into the file descriptor
  table for the process.
• Read, write etc. will use this user-space file
  descriptor.

9
(No Transcript)
10
The Directory Cache

• Speed up accesses to commonly used directories.
• Hash table- device number and directory's name
  to calculate the hash value
• Only short directory entries (up to 15 characters
  long) are cached. eg./usr/X11R6/bin
• Dcache replacement policy- two level LRU cache
  list.

11
VFS Inode Cache

• Is similar to Dcache in implementation.

12
Buffer Cache

• The Linux file systems regard block devices as a
  linear collections of blocks irrespective of the
  underlying physical disk's geometry.
• Device driver maps a particular block of its
  device into track, sector and cylinder etc.
• As blocks are accessed, they are put into the
  Buffer Cache
• Also helps manage the asynchronous interface with
  the block device drivers.

13
Buffer Cache

• Shared between all of the physical block devices.
• Block buffers within the cache are identfied by
  the device identifier and the block number.
• 2 functional parts
• lists of free block buffers (of different sizes)
• the cache (hash table).
• House-keeping of the buffer cache pdflush

14
VFS Inode

• Device, inode number
• unique within VFS.
• owner uid
• permission information
• i_op
• pointer to inode operations.
• dirty
• VFS inode has been written to (write back to the
  block device).

15
Implementing a filesystem

• struct file_system_type
• read_super
• called when a new instance of this file system is
  mounted.
• s_op pointer to a "struct super_operations"
  (next level of the filesystem implementation.)
• struct super_operations
• read_inode, write_inode, write_super ...
• read_inode (struct inode )
• fills the i_op member of the VFS inode
• Methods that can be performed on individual
  inodes.

16

• read_inode
• also fills the i_fop field with the struct
  file_operations
• struct inode_operations
• lookup called when the VFS needs to look up an
  inode in a parent directory.
• struct file_operations
• read, write, readdir
• open
• before open is called VFS copies i_fop field of
  the inode to the f_op field of the newly
  allocated file structure.

17
(No Transcript)