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Pertemuan 22Implementasi Sistem File

Matakuliah : T0316/sistem Operasi

Tahun : 2005

Versi/Revisi : 5

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa

akan mampu :• menunjukkan implementasi sistem file (C3)

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Outline Materi

• Contiguous Allocation• Linked list allocation• Linked list allocation using Table• I-Nodes• Implementasi directory• Disk space management

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File System Implementation

A possible file system layout

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Implementing Files (1)

(a) Contiguous allocation of disk space for 7 files

(b) State of the disk after files D and E have been removed

+ simplicity: only remember disk address & number of blocks in the file

read performance

– fragmentation

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Implementing Files (2)

Storing a file as a linked list of disk blocks

+ minimal fragmentation

– random access is slow

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Implementing Files (3)

Linked list allocation using a file allocation table (FAT) in RAM

+ minimal fragmentation, random access is easier

– entire table must be in memory all the time

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Implementing Files (4)

An example i-node

+ in memory when the file is open

– disk address may insufficient use the last address as the address of more disk block addresses

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Implementing Directories

The directory entries provide information needed to find the disk blocks

The information may be:

• the disk address of the entire file (contiguous)

• the number of first block (linked list)

•The number of I-node

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Implementing Directories (1)

(a) A simple directory (for MS-DOS/Windows)fixed size entries

disk addresses and attributes in directory entry

(b) Directory in which each entry just refers to an i-node (for UNIX)

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Implementing Directories (2)

• Two ways of handling long file names in directory– (a) In-line– (b) In a heap

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Shared Files (1)

File system containing a shared file

– consistency problem in update

disk blocks are listed in the data structure/I-node instead of directories

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use symbolic linking which contains the file’s path name

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Shared Files (2)

(a) Situation prior to linking

(b) After the link is created

(c)After the original owner removes the file

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Disk Space Management (1)

• Dark line (left hand scale) gives data rate of a disk• Dotted line (right hand scale) gives disk space efficiency• All files 2KB

Block size

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Disk Space Management (2)

(a) Storing the free list on a linked list(b) A bit map

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Disk Space Management (3)

Quotas for keeping track of each user’s disk use

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File System Reliability

• Backups

• Strategies for dumping disk to a tape– Physical dump

• From block 0 until last

– Logical dump• Dump all files and directories changed since given

base date

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File System Reliability (1)-- Logical dump

• A file system to be dumped– squares are directories, circles are files– shaded items, modified since last dump– each directory & file labeled by i-node number

File that hasnot changed

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File System Reliability (2)

Bit maps used by the logical dumping algorithm(a) Dump changed files and all directories, then

(b) Dump changed files and directories above them(c) + (d) (b)

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File System Reliability (3)

• File system states(a) consistent(b) missing block(c) duplicate block in free list(d) duplicate data block

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File System Performance (1)

The block cache data structures

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File System Performance (2)

• I-nodes placed at the start of the disk• Disk divided into cylinder groups

– each with its own blocks and i-nodes

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Log-Structured File Systems

• With CPUs faster, memory larger– disk caches can also be larger– increasing number of read requests can come from

cache– thus, most disk accesses will be writes

• LFS Strategy structures entire disk as a log– have all writes initially buffered in memory– periodically write these to the end of the disk log– when file opened, locate i-node, then find blocks