Chapter 4 File Systems What is a file

Chapter 4 : File Systems • • What is a file system? Objectives & user requirements Characteristics of files & directories File system implementation Directory implementation Free blocks management Increasing file system performance Ceng 334 - Operating Systems 1

File System • The collection of algorithms and data structures which perform the translation from logical file operations (system calls) to actual physical storage of information Ceng 334 - Operating Systems 2

Objectives of a File System • Provide storage of data and manipulation • Guarantee consistency of data and minimise errors • Optimise performance (system and user) • Eliminate data loss (data destruction) • Support variety of I/O devices • Provide a standard user interface • Support multiple users Ceng 334 - Operating Systems 3

User Requirements • Access files using a symbolic name • Capability to create, delete and change files • Controlled access to system and other users’ files • Control own access rights • Capability of restructuring files • Capability to move data between files • Backup and recovery of files Ceng 334 - Operating Systems 4

Files • Naming – Name formation – Extensions (Some typical extensions are shown below) Ceng 334 - Operating Systems 5

Files (Cont. ) • Structuring – (a) Byte sequence (as in DOS, Windows & UNIX) – (b) Record sequence (as in old systems) – (c) Tree structure (as in some mainframe Oses) Ceng 334 - Operating Systems 6

Files (Cont. ) • File types – Regular (ASCII, binary) – Directories – Character special files – Block special files • File access – Sequential access – Random access Ceng 334 - Operating Systems 7

Files (Cont. ) • File attributes – Read, write, execute, archive, hidden, system etc. – Creation, last access, last modification Ceng 334 - Operating Systems 8

Files (Cont. ) • File operations 1. Create 2. Delete 3. Open 4. Close 5. Read 6. Write Ceng 334 - Operating Systems 7. Append 8. Seek 9. Get attributes 10. Set Attributes 11. Rename 9

Directories • Where to store attributes – In directory entry (DOS, Windows) – In a separate data structure (UNIX) • Path names – Absolute path name – Relative path name – Working (current) directory • Operations – Create, delete, rename, open directory, close directory, read directory, link (mount), unlink Ceng 334 - Operating Systems 10

Directories & Files (UNIX) Disk A d 2 f 2 Linked Branch d 3 f 4 d 1 f 1 / Root Directory Working Directory f 3 f 5 d 4 Disk B d 5 f 6 d 6 f 7 • Working directory : d 2 • Absolute path to file f 2 : /d 1/d 2/f 2 • Relative path to file f 2 : f 2 Ceng 334 - Operating Systems 11

Physical Disk Space Management Sector Track Cylinder Heads • Each plate is composed of sectors or physical blocks which are laid along concentric tracks • Sectors are at least 512 bytes in size • Sectors under the head and accessed without a head movement form a cylinder Ceng 334 - Operating Systems 12

File System Implementation • Contiguous allocation • Linked list allocation using an index (DOS file allocation table - FAT) • i-nodes (UNIX) Ceng 334 - Operating Systems 13

Contiguous Allocation • The file is stored as a contiguous block of data allocated at file creation (a) Contiguous allocation of disk space for 7 files (b) State of the disk after files D and E have been removed Ceng 334 - Operating Systems 14

Contiguous Allocation (Cont. ) • FAT (file allocation table) contains file name, start block, length • Advantages – Simple to implement (start block & length is enough to define a file) – Fast access as blocks follow each other • Disadvantages – Fragmentation – Re-allocation (compaction) Ceng 334 - Operating Systems 15

Linked List Allocation • The file is stored as a linked list of blocks Ceng 334 - Operating Systems 16

Linked List Allocation (Cont. ) • Each block contains a pointer to the next block • FAT (file allocation table) contains file name, first block address • Advantages – Fragmentation is eliminated – Block size is not a power of 2 because of pointer space • Disadvantages – Random access is very slow as links have to be followed Ceng 334 - Operating Systems 17

Linked list allocation using an index (DOS FAT) FAT (File allocation table) 0 Disk size 1 EOF 2 Free 3 5 4 Free 5 7 6 Bad 7 1 …. . n Free Ceng 334 - Operating Systems 3 5 7 1 File blocks First block address is in directory entry 18

Linked list allocation using an index (Cont. ) • The DOS (Windows) FAT is arranged this way • All block pointers are in FAT so that don’t take up space in actual block • Random access is faster since FAT is always in memory • 16 -bit DOS FAT length is (65536+2)*2 = 131076 bytes Ceng 334 - Operating Systems 19

Problem • 16 -bit DOS FAT can only accommodate 65536 pointers (ie. , a maximum of 64 MB disk) • How can we handle large disks such as a 4 GB disk? Ceng 334 - Operating Systems 20

i (index)-nodes (UNIX) File mode Number of links UID GID File size Time created Time last accessed Time last modified 10 disk block numbers Single indirect block Double indirect block Triple indirect block Indirect blocks Ceng 334 - Operating Systems Data blocks 21

i-nodes (Cont. ) • Assume each block is 1 KB in size and 32 bits (4 bytes) are used as block numbers • Each indirect block holds 256 block numbers • First 10 blocks : file size <= 10 KB • Single indirect : file size <= 256+10 = 266 KB • Double indirect : file size <= 256*256 +266 = 65802 KB = 64. 26 MB • Triple indirect : file size <= 256*256 + 65802= 16843018 KB = ~16 GB Ceng 334 - Operating Systems 22

Directory Implementation • DOS (Windows) directory structure • UNIX directory structure Ceng 334 - Operating Systems 23

DOS (Windows) Directory Structure (32 bytes) 8 bytes 3 1 File name Ext A 10 Reserved 2 2 2 T D P 4 Size Attributes (A, D, V, S, H, R) Time of creation Date of creation Pointer to first data block Ceng 334 - Operating Systems 24

UNIX Directory Structure (16 bytes) 2 bytes I-node # Ceng 334 - Operating Systems 14 bytes File name 25

The Windows 98 Directory Structure • Extended MS DOS Directory Entry • An entry for (part of) a long file name Ceng 334 - Operating Systems 26

The Windows 98 Directory Structure An example of how a long name is stored in Windows 98 Ceng 334 - Operating Systems 27

Path Name Lookup : /usr/ast/mbox Root (/) i-node 245 Root directory file block 245 1. 1. . 4 bin 7 dev 14 lib 9 etc 6 usr i-node 60 of /usr/ast/mbox Blocks of file Ceng 334 - Operating Systems i-node 6 of /usr 132 /usr/ast directory file block 406 26. 6. . 60 mbox 92 books 81 src /usr directory file block 132 6. 1. . 19 prog 30 stu 51 html 26 ast 45 genc i-node 26 of /usr/ast 406 28

Two ways of handling long file names in a Directory In-line Ceng 334 - Operating Systems In a heap 29

Shared Files / d 1 f 1 d 2 f 3 • File f 2 is shared by two paths (users!) and there is one physical copy. • The directories d 1 & d 2 point to the same i-node with link count equal to 2 • Deletion is done by decrementing the link count. When it reaches zero the file is deleted physically Ceng 334 - Operating Systems 30

Disk Space Management Block size • Dark line (left hand scale) gives data rate of a disk • Dotted line (right hand scale) gives disk space efficiency • All files 2 KB Ceng 334 - Operating Systems 31

How to Keep Track of Free Disk Blocks • Linked list of disk blocks • Bit maps • Indexing as used in DOS FAT Ceng 334 - Operating Systems 32

Linked List of Disk Blocks • Allocation is simple. • Delete block number from free blocks list Ceng 334 - Operating Systems 33

Bit Maps • The bit map is implemented by reserving a bit string whose length equals the number of blocks • A ‘ 1’ may indicate that the block is used and ‘ 0’ for free blocks • If the disk is nearly full then the bit map method may not be as fast as the linked list method Ceng 334 - Operating Systems 34

Increasing File System Performance • Disks (floopies, hard disks, CD ROMS) are still slow when compared to the memory • Use of a memory cache may speed the disk transfers between disk and process • Blocks are read into the cache first. Subsequent accesses are through the cache • Blocks are swapped in & out using replacement algorithms such as FIFO, LRU • System crashes may cause data loss if modified blocks are not written back to disk Ceng 334 - Operating Systems 35

Where to Put the Current “File Position” Field • The file position field is a 16 or 32 bit variable which holds the address of the next byte to be read or written in a file – Put it in the i-node – Put it in process table Ceng 334 - Operating Systems 36

File Position Field in i-node • If two or more processes share the same file, then they must have a different file position • Since i-node is unique for a file, the file position can not be put in the i-node Ceng 334 - Operating Systems 37

File Position Field in Process Table • When a process forks, both the parent and the child must have the same file position • Since the parent and the child have different process tables they can not share the same file position • So, we can not put in process table Ceng 334 - Operating Systems 38

Solution • Use an intermediate table for file positions Process tables File positions table parent position child Ceng 334 - Operating Systems i-node of file 39