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14: Filesystem Examples:CD-ROM, MS-DOS, Unix

Mark Handley

CD Filesystems

ISO 9660Rock Ridge Extensions

Joliet Extensions

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ISO 9660: CD-ROM Filesystem

CD is divided into logical blocks of 2352 bytes. 2048 bytes of payload per block, after preambles, error

correction, etc CD-ROM starts with 16 unspecified blocks, Then one block containing primary volume descriptor

system id, volume id, publisher id, etc where to find root directory

Each directory consists of a variable number of entries. A flagindicates the last one.

No more than 8 levels of directory nesting.

An ISO 9660 Directory Entry

Files are contiguous, so only need starting block and file size. Separate bytes for year, month, day, hour, min, sec, timezone. L: filename length. Filename: 8.3 uppercase format for basename.ext followed by

; then version (1 or 2 binary bytes). Last: optional system use field.

Bytes

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CD-ROM Extensions

Rock Ridge Designed to allow Unix

filesystems to be stored, Uses “system use” field, so

is backward compatible. PX - POSIX attributes

permissions, etc PN - device numbers SL - symbolic link NM - alternative name CL, PL, RE - relocation TF - time stamps

Joliet Designed to allow

Microsoft filesystems to bestored on CD. Long names: 128 chars Unicode Directory nesting

greater than 8 levels Directory names with

extensions.

MS-DOS Filesystem

Extension for larger disksExtension for better metadata

Extension for unicode and longer filenames.

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MS-DOS

Uses a FAT filesystem.Derived from earlier CP/M filesystem

3 versions, depending on number of bits in blockaddress:FAT-12 has 12-bit block addressesFAT-16 has 16-bit block addressesFAT-32 has 28-bit block addresses

MS-DOS

Original FAT-12 used 12-bit block addresses with 512 byteblocks: 2MB max partition, FAT table requires 4096 entries of 2

bytes each in memory. Good for floppy drives and 640K RAM.

Use of bigger block sizes, and up to four partitions per drivestretched FAT-12 to 64MB disks.

FAT-16 introduced with 16-bit block addresses. Max 64K blocks of 32KBytes each => 2GB per partition. FAT requires 128KBytes of RAM.

Windows 95-SE introduced FAT-32.

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MS-DOS: Maximum Partition Sizes

MS-DOS Directory Entry

Fixed size: 32 bytes. Attributes: read-only, needs to be archived, hidden, system file. Date and Time file was last modified

± 2 seconds accuracy, rollover in 2108 First disk block of file - remainder found via FAT. Max file size: 2GB

Base name Ext file sizelast moddate/time

8 3 1 4 410 2

Attributes First blocknumber

Reserved

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The Windows 98 Directory Entry

The extended MS-DOS directory entry used in Windows 98:

Creation date/time, separate from Last Modified date/time,and accurate to 10ms (via sec field)

Last Access date (but not time) NT compatible filesystem case sensitivity. Extra 16 bits block index needed for FAT-32.

Base name ExtNT

file sizelast moddate/time

Creationdate/time

lastacces

8 3 1 4 211 4 42 2

Attributes Sec upper 16 bitsof first file block

lower 16 bitsof first file block

Windows 98 (Extension) Directory Entry

An entry for part of a long file name in Windows 98 Characters are unicode, so take 2 bytes each. A complete directory entry has one 8.3 ASCII MS-DOS

compatible base directory entry and multiple extensionentries containing the long unicode filename.

Checksum is for backwards compatibility: detects file deletion/readdition on old systems.

0 2 characterslast moddate/time

Creationdate/time

lastacces

10 1 1211 42

Attributes(0x0F)

Checksum

06 characters5 characters

1

Sequence

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Windows 98: Storing Long Filenames

An example of how a long name is stored in Windows 98

Base name Ext A SNT

file sizelast moddate/time

Creationdate/time

lastacces

8 3 1 4 211 4 42 2

high lo

0 2 characterslast moddate/time

Creationdate/time

lastacces

10 1 1211 42

06 characters5 characters

1

A SNT

file sizelast moddate/time

Creationdate/time

lastacces

high lo

0last moddate/time

Creationdate/time

lastacces 01

0last moddate/time

Creationdate/time

lastacces 02

0last moddate/time

Creationdate/time

lastacces 067

THI TXT~1ISS

T i sh

y l o n

m !e

i as

g f i l

v

e

e r

n astoredfirst

storedlast

“This is a very long filename!.TXT” == THISIS~1.TXT

Unix Filesystem

Version 7 FilesystemBSD FFS extensions

Linux filesystem structure

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UNIX File System

Disk layout in classical UNIX systems

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The UNIX V7 File System:

A Directory Entry

System 7 released January 1979 Filenames limited to 14 characters. All metadata is stored in the i-node, not in the directory.

The UNIX V7 File System:

i-nodes and indirect blocks

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The UNIX V7 File System:

Locating a File

The steps in looking up /usr/ast/mbox

The UNIX File System:

A Single Directory Hierarchy

Before mounting.

/

a

c

b

/

d

p q r

x y z

/

a

c d

p q r

x y z

bb

Hard Disk CD-ROM Unified Filesystem

After mounting

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System Calls for File Management

s is an error code fd is a file descriptor position is a file offset

UNIX File System (2)

Structure of an i-node

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The stat() System Callint stat(const char *path, struct stat *sb);struct stat { dev_t st_dev; /* device inode resides on */ ino_t st_ino; /* inode's number */ mode_t st_mode; /* inode protection mode */ nlink_t st_nlink; /* number or hard links to the file */ uid_t st_uid; /* user-id of owner */ gid_t st_gid; /* group-id of owner */ dev_t st_rdev; /* device type, for special file inode */ struct timespec st_atimespec; /* time of last access */ struct timespec st_mtimespec; /* time of last data modification */ struct timespec st_ctimespec; /* time of last file status change */ off_t st_size; /* file size, in bytes */ quad_t st_blocks; /* blocks allocated for file */ u_long st_blksize; /* optimal file sys I/O ops blocksize */ u_long st_flags; /* user defined flags for file */ u_long st_gen; /* file generation number */ };

System Calls for Directory Management

s is an error code dir identifies a directory stream dirent is a directory entry

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UNIX File System:

File Descriptors

The relation between the file descriptor table, the openfile description, the inode, and the file.

BSD Filesystem Directories

A BSD directory with three files The same directory after the file voluminous has been

removed

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The Linux Ext2 File System

Directory Structure is very similar to BSD FFS. Disk layout differs: block groups rather than cylinder

groups.