Lecture 17
File Systems
Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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Device Special File
• Corresponds to device on system
• Located in /dev
• Some are real, e.g.,
• Mouse
• Keyboard
• Disk
• Others are virtual
• No actual hardware
• Kernel provides abstract device
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Device Driver
• Kernel code that implements operations specific to hardware
• API is fixed, e.g.,
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open()
close()
read()
write()
etc.
• Allows universality of I/O
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Types of Devices
• Character devices- handles data character by character, e.g.,
• Terminal
• Keyboard
• Block devices- handles data a block at a time, e.g., disk
• Size depends on device, typically 512 bytes (or multiple thereof)
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udev Program
• Relies on sysfs file system
• Exports information about devices into user space
• Uses a pseudo file system mounted under /sys
• E.g., when plug in usb flash driver...
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Device ID
• Major ID
• Identifies general class of device
• Used by kernel to locate driver
• Minor ID
• Identifies a particular device within general class
• Can be displayed with ls –l command
• Device IDs recorded in i-node of device file
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Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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Disks and Partitions
• Regular files and directories usually reside on hard disk devices
• Can also exist on CD-ROMS, flash drivers, virtual disks
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Disk Drives
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Hard disk is a mechanical device
Information stored on set of concentric circles called tracks
Tracks divided into sectors
Sectors contain series of physical blocks
• Typically 512 bytes in size (or multiples thereof)
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Disk Drives
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Disk access is SLOW (on order of milliseconds)
Seek time- move to appropriate track
Rotation latency- wait until sector moves under disk head
Transfer time- transfer bytes
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Partitions
• Disk divided into one (or more) non-overlapping partitions
• Each partition treated as separate device /dev
• Usually contain:
• file system- hold regular files and directories
• data area- accessed as a raw-mode device
• swap area- used by kernel for memory management
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Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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File Systems
• Organized collection of regular files and directories
• mkfs command makes a file system
• Linux supports variety of file systems:
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ext2
minix, system V, BSD (native UNIX)
FAT, FAT32, NTFS (Microsoft)
HFS (Apple)
NFS (network file systems)
ext3, ext4, JFS, XFS (journaling file systems)
...
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ext2 File System
• Second Extended File System
• Use has declined in favor of journaling file systems
• 5000+ lines of C
• Used to explain file system concepts
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Logical Block
• Basic unit for allocating file system space
• Multiple contiguous physical block on disk device
• ext2 logical block size is 1024, 2048, or 4096 bytes
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File System Structure
• File system contains following parts:
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Boot block
Superblock
i-node table
Data blocks
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File System Boot Block
• ALWAYS first block in file system
• Not used by file system
• Contains info used to boot operating system
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File System Superblock
• Single block, immediately following boot block
• Contains parameter info about file system
• Size of i-node table
• Size of logical blocks
• Size of file system
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File System i-node Table
• Each file / dir in file system has unique entry in i-node table
• Entry records various information about file
• Also known as i-list
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File System Data Blocks
• Majority of space in file system
• Form files and directories of files system
• (Where actual bytes reside on disk)
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Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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i-nodes
• i-node stands for index node
• Each row of i-node table contains one i-node
• One i-node for each file in file system
• Identified sequentially (numerically) by location in i-node table
• “i-node number”
• UNIX> ls –li
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Information Maintained by i-node
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File type
Owner
Group
Access permissions
Timestamps: modified, access, status change
Number of hard links to file
Number of blocks allocated to file
Pointers to data blocks of file
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Files and Data Blocks
• ext2 file system does NOT store data blocks in sequential order
• Does try to store data blocks “close together”
• Kernel maintains set of pointers in i-node to locate data blocks
• Reduces risk of disk fragmentation
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i-node Data Block Pointers
• Each i-node contains 15
pointers
• Pointers 0-11: first 12 data
blocks of file
• Pointer 12: pointer to block
of pointers (single indirect)
• Pointer 13: double indirect
pointer
• Pointer 14: triple indirect
pointer
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i-node Data Block Pointers
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i-node Data Block Pointers
• Why such a complex system of pointers to pointers to
pointers...?
• Allows i-node structure to be fixed size (always 15 pointers)
• Allows file system to store files non-contiguously
• Allows any size file
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Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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Virtual File System (VFS)
• File systems (e.g., ext2, FAT) have different implementations
• Kernel provides abstraction (VFS)
• Provides generic interface for file system operations
• Programs only need to understand VFS (e.g., open(), close(), etc.)
• Each file system implements VFS interface
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Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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July 24, 2016
Journaling File Systems
• ext2 file system is “traditional”
• Suffers classic limitation: after system crash, file system consistency check
(fsck) must be performed to insure integrity
• At time of crash, file update may have been only partially completed
• Consistency check must examine entire file system!! (slow)
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Journaling File Systems
• Journaling eliminates need for consistency check after crash
• Journaling logs metadata to special on-disk journal before file
operations are carried out
• If system crashes, log used to rapidly redo incomplete updates
• E.g., ext3
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Outline
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Devices
Disks and Partitions
File Systems
i-nodes
Virtual File System
Journaling
Mounting
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Single Directory Hierarchy and Mount Points
• ALL files from file systems reside under single directory tree
• Base of tree is root directory ( ‘/‘ )
• Other file systems mounted under root directory and appear
as subtrees within overall hierarchy
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Single Directory Hierarchy and Mount Points
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Mount
• To mount a device:
UNIX> sudo mount device directory
• Attach device into directory hierarchy at specified directory
• directory known as device’s mount point
• Use umount command to detach device
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Mount
UNIX> mount
/dev/disk0s2 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
map -hosts on /net (autofs, nosuid, automounted, nobrowse)
map auto_home on /home (autofs, automounted, nobrowse)
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• List file systems currently attached (e.g., on my Mac)
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/proc/mounts
• Look up info about mounted devices
• Interface of kernel data structures
• Example line from /proc/mounts
/dev/sda9
/boot ext3
rw
0
0
mount point
file system type
name of
mounted device
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always 0
(unused in
/proc/mounts)
mount flags
(e.g., read, write)