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

Transcription

1 17: Filesystem Examples: CD-ROM, MS-DOS, Unix Mark Handley CD Filesystems ISO 9660 Rock Ridge Extensions Joliet Extensions 1

2 ISO 9660: CD-ROM Filesystem CD is divided into logical blocks of 2352 bytes 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 flag indicates the last one. No more than 8 levels of directory nesting. An ISO 9660 Directory Entry Bytes 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. 2

3 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 be stored on CD. Long names: 128 chars Unicode Directory nesting greater than 8 levels Directory names with extensions. MS-DOS Filesystem Extension for larger disks Extension for better metadata Extension for unicode and longer filenames. 3

4 MS-DOS Uses a FAT filesystem. Derived from earlier CP/M filesystem 3 versions, depending on number of bits in block address: FAT-12 has 12-bit block addresses FAT-16 has 16-bit block addresses FAT-32 has 28-bit block addresses Original FAT-12 with 512 byte blocks: MS-DOS Original FAT-12 used 12-bit block addresses with 512 byte blocks: 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 drive stretched 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. 4

5 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 5

6 The Windows 98 Directory Entry Base name Ext N T Creation date/time last acces last mod date/time file size Attributes Sec upper 16 bits of first file block lower 16 bits of first file block 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. Windows 98 (Extension) Directory Entry characters 0 Creation last last mod 6 characters date/time acces date/time 0 2 characters Sequence Attributes (0x0F) Checksum 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 extension entries containing the long unicode filename. Checksum is for backwards compatibility: detects file deletion/readdition on old systems. 6

7 Windows 98: Storing Long Filenames N Creation last last mod Base name Ext A S high lo file size T date/time acces date/time characters 0 Creation last last mod 6 characters date/time acces date/time 0 2 characters T H I S I S ~ 1 T X T 1 T h i s A N T 0 S Creation last date/time acces high Creationlast date/time i s acces a Creationlast last mod date/time last mod v date/time last mod file size 2 y l o n 0 An example of how a long date/time g 0 name acces f i l e n a is stored date/time in Windows 98 Creationlast 67 m e! 0 0 date/time acces last mod date/time This is a very long filename!.txt == THISIS~1.TXT lo 0 e r stored last stored first Unix Filesystem Version 7 Filesystem BSD FFS extensions Linux filesystem structure 7

8 UNIX File System Disk layout in classical UNIX systems 8

9 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 9

10 The UNIX V7 File System: Locating a File The steps in looking up /usr/ast/mbox The UNIX File System: A Single Directory Hierarchy Separate file systems After mounting (a) (b) (a) Before mounting. (b) After mounting 10

11 System Calls for File Management s is an error code fd is a file descriptor position is a file offset UNIX File System (2) Directory entry fields. Structure of the i-node 11

12 The stat() System Call int 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 12

13 UNIX File System: File Descriptors The relation between the file descriptor table, the open file 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 13

14 The Linux Ext2 File System Directory Structure is very similar to BSD FFS. Disk layout differs: block groups rather than cylinder groups. 14