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 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 The VFS Inode 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

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 from vfsmntlist

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