Digital UNIX Internals II 5 - 1 UFS

UNIX File System (UFS)

Chapter Five

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Topics

• Background: Old System V File System• On Disk Format

– inodes– fragments– block allocation

• Consistency Issues

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Features

• Traditional BSD Local File System• Compatible with Berkeley 4.3 Tahoe release• Digital Enhancements

– increased file and path sizes– file block clustering– File on File Layer for STREAMS– Unified Buffer Cache Integration– Support for Enhanced File Attributes

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History: The old System V File System Layout

DataInodeTable

SuperBlock

BootBlock

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UNIX File System Layout

AlternateSuperBlock

BootBlock

SuperBlock

CylinderGroupBlock

InodeTable

Data DataAlternate

SuperBlock

CylinderGroupBlock

InodeTable

Data

Cylinder Group 0 Cylinder Group N

Partition

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UFS Block Types

• bootblocks• superblocks• inodes• data blocks• cylinder group information

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Inodes

• State of an individual file• disk inode

– stored on disk– sized to fit evenly in a disk block

• incore inode– in-memory– disk inode information plus information related to being a

currently open file– for UFS, vnode v_data private is an incore inode

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Disk Inode

ModeLink

CountUID GID Size

AccessDate

ModifyDate

CreateDate

15 Pointers

ActualBlockCount

Gener-ation

Number

0 0 0 0 0 0 0 0 0 0 0 0 1 2 3

Data Block PointersLevel of

Indirection

DataBlock

IndirectionBlock

IndirectionBlock

IndirectionBlock

(not implemented)

DataBlock

IndirectionBlock

IndirectionBlock

DataBlock

SUID

SGID

StickyFile type User Group Other

0 127

Common to all File Systems Specific to this File System

DataBlock

IndirectionBlock

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Inode Numbers

• Inodes numbered from 1– Position #1,

• originally an inode for bad data blocks, no longer used for that purpose.

– Position #2• inode for the root directory of the file system

• UFS file identity is <device, inumber>

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Small Blocks Require More Overhead

X X X X X X X X

X

FILE 1

FILE 2

Large Blocks Waste SpaceX

FILE 1

FILE 2

X X X X X X X X

Large Blocks and Small Fragments

FILE 1

FILE 2

X X X X X X X X

X

Fragments (1)

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Fragments (2)

• UFS Blocks are 8192 Bytes– good for modern disk drives– bad for small files

• space wasted with internal fragmentation

• Fragment– Subdivision of a Block– 1024 Bytes– Consecutive fragments are use for the last partial block of

small files

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Fragment (3)

• Cylinder Group Block has one bit set for each fragment in the cylinder group

Block #

free bits 01101011 11111111 00010000

117 118 119

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Disk Block Allocation

• Whenever Possible – First 96KB Direct Block Allocation

• in cylinder containing inode

– Each Additional 2 Meg• cylinder groups with "lots" of free space

– Allocate consecutive disk blocks

• Works well until disk reaches 90% full

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Inode Allocation

• Regular Files– in cylinder group of parent directory

• Directories– in different cylinder group than parent directory

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Directories in UFS

• Collection of inode:file_name pairs– four byte inode number– two byte length of entry– two byte length of name– null terminated name– padding to next four byte boundary

• Deleted entries become padding for previous• Entries must fit in a 512-byte sector so disk write is

atomic

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Symbolic Links

• Hard Links: ln– new directory entry added referencing an existing inode– link count in existing inode incremented

• Soft Links: ln -s– new directory entry added referencing a new inode– new inode is of type sym link

• if link path short enough

– stored in data block map area of inode

– referred to as a fast link

• else

– link path stored in a data block

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UFS Consistency

• If Machine Crashes– writes to data blocks may be lost

• Disaster if the lost block – is an indirect block containing the disk addresses of newly

written data blocks already on disk.– is a directory block containing new file references newly

created files with written inodes. – contains free list information about newly allocated blocks.

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UFS Solution

• Some Writes are Synchronous– Any changed block that refers to other blocks

• 30 second update process sync's all mounted file systems

• Directory entries are not written across sector boundaries• fsck utility after reboot

– must know usual order of UFS synchronous writes

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Source References

• kernel/ufs/fs.h– definitions of super block (struct fs) and cylinder groups

(struct csum)

• kernel/ufs/dinode.h– definition on disk inode (struct dinode)

• kernel/ufs/inode.h– definition of in-memory inode (struct inode)

• kernel/ufs/ufs_vnops.c– implementation of vnode ops for ufs

• kernel/ufs/ufsmount.h– definition of the ufs mount structure

• kernel/ufs/ufs_vfsops.c– implementation of the ufs vfs ops.