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The File System

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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)

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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

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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

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Physical Layout of EXT2

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The EXT2 Inode

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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

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The EXT2 Inode

• Timestamps

– creation and modification

• Datablocks

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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

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The EXT2 Superblock

• Basic size and shape of the file system

– Blocks per Group

– Free Blocks

– Free Inodes

– First Inode

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The EXT2 Group Descriptor

• Block Bitmap

• Inode Bitmap

• Inode Table

• Free blocks count, Free inodes count, Used directory count

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EXT2 Directory

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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

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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

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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

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The Virtual File System(VFS)

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The VFS Superblock

• Device

• Inode pointers

• Blocksize

• Superblock operations

• File System Type

• File System specific

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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

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Registering the File Systems

• Build Linux kernel : supported file systems

• Build file systems as modules

– load by ismod

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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

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A Mounted File System

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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

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Speedup Access

• VFS Inode Cache

• Directory Cache

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The Buffer Cache

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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,

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The States of the Cache

• Share

– Shared buffers,

• Unshared

– Buffers that were once shared but which are now not shared, clean :Unused, new buffers,

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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)

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The /proc File System

• It does not really exist

• Presents a user readable windows into the kernel’s inner workings

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Devices Special Files

• Hardware devices

• character and block devices

• device drivers

– major number, minor number