The File System

1. The File System

2. Linux File System • Linux supports 15 file systems • ext, ext2, xia, minix, umsdos, msdos, vfat, proc, smb, ncp, iso9660, sysv, hpfs, affs and ufs • The separate file systems are combined into a single hierarchical tree structures • mount on a directory (mount point)

3. Virtual File System • Disks are initialized into logical partitions • Each partition may hold a single file system • EXT2 • The real file systems are separated from the operating system by an interface layer: Virtual File System

4. The Second Extended File System (EXT2) • File : data blocks • inode : describe which blocks the data within a file occupies, access rights, modification time,… • directory : special file which contains pointers to the inodes

5. Physical Layout of EXT2

6. The EXT2 Inode

7. The EXT2 Inode • Mode • what this inode describes (file, directory, symbolic link, FIFO,…) and the permissions • Owner Information • user and group ids of the owners • Size

8. The EXT2 Inode • Timestamps • creation and modification • Datablocks

9. The EXT2 Superblock • Basic size and shape of the file system • Magic Number :0xEF53 • Revision Level • Mount Count and Maximum Mount Count • Block Group Number • Block Size • Block per Group

10. The EXT2 Superblock • Basic size and shape of the file system • Blocks per Group • Free Blocks • Free Inodes • First Inode

11. The EXT2 Group Descriptor • Block Bitmap • Inode Bitmap • Inode Table • Free blocks count, Free inodes count, Used directory count

12. EXT2 Directory

13. Finding a File in an EXT2 File System • /home/rusling/.cshrc • system parse the filename a directory at a time until we get the file

14. Changing the Size of a File in an EXT2 File System • Lock EXT2 Superblock • Check if there are preallocated blocks • EXT2 allocate new block • data block after the last block of the file • data blocks within 64 blocks of the idea block • data block in the same Block Group

15. Changing the Size of a File in an EXT2 File System • All of the other Block Groups in turns (allocate a cluster of eight blocks) • Update the Block Group’s block bitmap and allocate a data buffer in the buffer cache • Mark the superblock as “dirty” and unlock

16. The Virtual File System(VFS)

17. The VFS Superblock • Device • Inode pointers • Blocksize • Superblock operations • File System Type • File System specific

18. Device Inode Number Mode User id times block size inode operations a pointer to a block of routine addresses count lock dirty file system specific The VFS Inode

19. Registering the File Systems • Build Linux kernel : supported file systems • Build file systems as modules • load by ismod

20. Mounting a File System • $mount -t iso9600 -o ro /dev/cdrom /mnt/cdrom • Search for the file system types( iso9600) • Allocate a VFS superblock and pass it the mount information to the superblock read routine

21. A Mounted File System

22. Umount a File System • Check whether someone is using the FS • Check if the FS is dirty • write back • Return VFS superblock to kernel’s pool • vfsmount is unlinked fromvfsmntlist

23. Speedup Access • VFS Inode Cache • Directory Cache

24. The Buffer Cache

25. The States of the Cache • Clean :Unused, new buffers, • Locked • Buffers that are locked, waiting to be written, • Dirty • Dirty buffers. These contain new, valid data, and will be written but so far have not been scheduled to write,

26. The States of the Cache • Share • Shared buffers, • Unshared • Buffers that were once shared but which are now not shared, clean :Unused, new buffers,

27. The bdflush Kernel Daemon • The bdflush kernel daemon is a simple kernel daemon that provides a dynamic response to the system having too many dirty buffers (default :60%) • The value can be seen and changed using update command (a daemon)

28. The /proc File System • It does not really exist • Presents a user readable windows into the kernel’s inner workings

29. Devices Special Files • Hardware devices • character and block devices • device drivers • major number, minor number