SISTIM OPERASI (Operating System) IKI-20230 File System Ch. 10 Johny Moningka (moningka@cs.ui.ac.id) Fakultas Ilmu Komputer Universitas Indonesia Semester 2000/2001 Ch. 10: File-System Interface File Concept Access :Methods Directory Structure Protection Consistency Semantics File System JM-2000/v1.1/2 File Concept Contiguous logical address space Types: Data • numeric • character • binary Program File System JM-2000/v1.1/3 Aspek Sudut Pandang User Media penyimpanan data/informasi: magnetic disk, magnetic tape, dan optical disk Untuk kenyamanan user OS menyediakan sudut pandang lojik yang sama bagi semua media dengan abstraksi unit-unit storage lojik yaitu file File System JM-2000/v1.1/4 Aspek Isi dan Struktur Isi File Representasi program atau data yang terekam dalam secondary storage Struktur file Bebas maupun berformat Secara umum file merupakan deretan bit, byte, baris, atau record yang artinya didefinisikan sendiri oleh user perancangnya File System JM-2000/v1.1/5 Atribut-atribut File Atribut dari file yang berlaku bebas dari user, proses maupun sistem Minimal file memiliki atribut nama Atribut-atribut lain: type informasi yang tersimpan, lokasi dalam device, ukuran dari data (byte, word, block), proteksi untuk mengendalikan akses, waktu (create, modifikasi terakhir, akses terakhir) dan identifikasi ownership File System JM-2000/v1.1/6 File Structure None - sequence of words, bytes Simple record structure Complex Structures Lines Fixed length Variable length Formatted document Relocatable load file Can simulate last two with first method by inserting appropriate control characters. Who decides: Operating system Program File System JM-2000/v1.1/7 File Attributes Name – only information kept in human-readable form. Type – needed for systems that support different types. Location – pointer to file location on device. Size – current file size. Protection – controls who can do reading, writing, executing. Time, date, and user identification – data for protection, security, and usage monitoring. Information about files are kept in the directory structure, which is maintained on the disk. File System JM-2000/v1.1/8 Operasi-operasi File Enam operasi dasar yang berkaitan dengan manajemen file sistem: Create file Write file Read file Reposition dalam file Delete file Truncate file File System JM-2000/v1.1/9 Add. Operasi-operasi File Create file: (1) menemukan free space; (2) entry baru dibuat dalam tabel direktori yang mencatat nama dan lokasi; serta (3) ukuran yang diinisialisasi 0 Write file: (1) OS melihat ke direktori untuk mencari lokasinya dalam disk; (2) melakukan transfer dari memori ke lokasi dalam disk (suatu pointer digunakan sebagai penunjuk lokasi penulisan berikutnya); dan (3) entry dalam direktori di update File System JM-2000/v1.1/10 Add. Operasi-operasi File Read file: Reposition dalam file: (1) OS melihat ke direktori untuk mencari entry yang dimaksud, (2) pointer di set dengan harga (lokasi) tertentu yang diberikan Delete file: OS melakukan hal yang sama dengan penulisan file kecuali operasinya membaca dari lokasi dalam disk ke dalam memori (1) OS melihat ke direktori mencari entry dengan nama yang dimaksud; (2) kemudian membebaskan space yang teralokasi; (3) serta menghapus entry tsb Truncate file: sama dengan menghapus file kecuali entry tidak dihapuskan tapi ukuran file diisi 0 File System JM-2000/v1.1/11 Operasi-operasi File Lain Operasi-operasi lain pada dasarnya dilakukan dengan kombinasi operasi-operasi dasar tadi, contoh: Append file Rename file Get atribut file Set atribut file File System JM-2000/v1.1/12 File Types – name, extension F ile T ype E xe cu ta b le O b je ct S o u rce co d e U sua l extension e xe , co m , b in o r none o b j, o F unction re a d y-to -ru n m a c h in e la n g u a g e p ro g ra m co m p lie d , m a ch in e la n g u a g e , n o t lin ke d so u rce co d e in va rio u s la n g u a g e s co m m a n d s to th e co m m a n d in te rp re te r te xtu a l d a ta d o c u m e n ts B a tch c, p , p a s, 1 7 7 , a sm , a b a t, sh T e xt txt, d o c W o rd p ro ce ss o r w p , te x, rrf, e tc. L ib ra ry lib , a va rio u s w o rd -p ro c e sso r fo rm a ts lib ra rie s o f ro u tin e s P rin t o r vie w p s, d vi, g if A S C II o r b in a ry file A rch ive a rc, zip , ta r re la te d file s g ro u p e d in to o n e file , so m e tim e s co m p re ss e d . File System JM-2000/v1.1/13 Open File Table Status file open terjadi saat reference pertama kali pada file dilakukan OS memelihara suatu tabel mencatat sejumlah informasi file-file yang open: File pointer: offset pada file yang akan diakses File open count: berapa banyak yang meng-open file tsb Disk location: lokasi blok pertama file dl disk File System JM-2000/v1.1/14 Pemanggilan Open File Table Beberapa sistem memungkinkan open & close dilakukan secara implisit dari proses (artinya: OS yang menangani sendiri) Sebagian sistem mengharuskan proses melakukan pemanggilan system call open/ close tersebut File System JM-2000/v1.1/15 Proses Open File Table Saat pemanggilan open, OS melakukan: mencari dalam direktori entry yang sesuai dengan nama yang diberikan jika belum ada, OS membuat entry baru dalam open file table dan menyalin informasi dalam directori entry tsb ke dalam entry open file table yang baru dibuat me-return ke proses pemaggil pointer entry dalam open file table untuk digunakan sebagai file handler dalam setiap operasi berikutnya File System JM-2000/v1.1/16 Open File Table Dua Level Dalam lingkungan multiuser operasi open/close lebih rumit di mana beberapa user dapat mengopen file yang sama pada saat yang bersamaan OS menggunakan dua level tabel internal open file table per proses open file table dari sistem File System JM-2000/v1.1/17 Open File Table per Proses Masing-masing proses memiliki tabel sendiri yang menyimpan informasi dari file-file yang dipakai Tabel-tabel per proses ini memiliki pointer ke open file table sistem File System JM-2000/v1.1/18 Open File Table dari Sistem Berisikan informasi yang independen terhadap proses: lokasi file dalam disk, tanggal akses, ukuran file open count: jumlah proses yang sedang mengakses file File System JM-2000/v1.1/19 Mekanisme Jika file sudah diopen oleh suatu proses, proses lain yang meng-open dilakukan dengan menambah entry baru pada tabel proses dengan file pointer baru, dan pointer ke entry ybs dalam tabel sistem dan open count bertambah satu Saat eksekusi close, open count berkurang satu dan bila menjadi 0 maka entry dihapus File System JM-2000/v1.1/20 Metoda Akses Sequential Access Direct Access (random access) Akses dilakukan dengan satu arah pembacaan/penulisan (dari awal hingga akhir) jika ingin mundur maka perlu dilakukan rewind Akses dilakukan bisa pada posisi mana saja dalam file Metoda lain Abstraksi lebih tinggi dari direct access Index file & relative file • Tabel Informasi index: record dan pointer ke file direct access. File System JM-2000/v1.1/21 Access Methods Sequential Access read next write next reset no read after last write (rewrite) Direct Access read n write n position to n read next write next rewrite n n = relative block number File System JM-2000/v1.1/22 Directory Structure Directory: A collection of nodes containing information about all files. Both the directory structure and the files reside on disk. Backups of these two structures are kept on tapes. File System JM-2000/v1.1/23 Structure Directory Files F1 F2 F3 F4 Fn File System JM-2000/v1.1/24 Information in a Device Directory Name Type Address Current length Maximum length Date last accessed (for archival) Date last updated (for dump) Owner ID (who pays) Protection information (discuss later) File System JM-2000/v1.1/25 Directory Operations Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system File System JM-2000/v1.1/26 Logical Organizations Efficiency – locating a file quickly. Naming – convenient to users. Two users can have same name for different files. The same file can have several different names. Grouping – logical grouping of files by properties, (e.g., all Pascal programs, all games, …) File System JM-2000/v1.1/27 Single-Level Directory A single directory for all users. Naming problem Grouping problem File System JM-2000/v1.1/28 Two-Level Directory Separate directory for each user. Path name Can have the saem file name for different user Efficient searching File System JM-2000/v1.1/29 Tree-Structured Directories File System JM-2000/v1.1/30 Tree-Structured Directories Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list File System JM-2000/v1.1/31 Tree-Structured Directories Absolute or relative path name Creating a new file is done in current directory. Delete a file rm <file-name> Creating a new subdirectory is done in current directory. mkdir <dir-name> Example: if in current directory /spell/mail mkdir count File System JM-2000/v1.1/32 mail prog copy prt exp count “mail” deleting the entire subtree rooted by “mail”. Deleting File System JM-2000/v1.1/33 Acyclic-Graph Directories Have shared subdirectories and files. File System JM-2000/v1.1/34 Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list dangling pointer. Solutions: Backpointers, so we can delete all pointers. Variable size records a problem. Backpointers using a daisy chain organization. Entry-hold-count solution. File System JM-2000/v1.1/35 General Graph Directory File System JM-2000/v1.1/36 General Graph Directory (Cont.) How do we guarantee no cycles? Allow only links to file not subdirectories. Garbage collection. Every time a new link is added use a cycle detection algorithm to determine whether it is OK. File System JM-2000/v1.1/37 Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List File System JM-2000/v1.1/38 UNIX: protection Mode of access: read, write, execute Three classes of users RWX a) owner access 7 111 RWX b) groups access 6 110 RWX c) public access 1 001 Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. File System JM-2000/v1.1/39 owner chmod Attach group 761 a group to a file chgrp public game G game File System JM-2000/v1.1/40