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Chapter 11: File System Implementation

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1 Chapter 11: File System Implementation

2 Outline File-System Structure File-System Implementation
Directory Implementation Allocation Methods Free Space Management Efficiency and Performance Recovery Log Structured File Systems

3 File System Structure File structure
Logical storage unit Collection of related information File system resides on secondary storage (disks) File system organized into layers File control block Storage structure consisting of information about a file

4 Layered File System

5 A Typical File Control Block

6 File System Structures
File system structures provided by the OS File open File read

7 Virtual File Systems Virtual File Systems (VFS) provide an object-oriented way of implementing file systems VFS allows the same system call interface (the API) to be used for different types of file systems The API is to the VFS interface, rather than any specific type of file system.

8 Schematic View of Virtual File System

9 Directory Implementation
Linear list of file names with pointer to the data blocks simple to program time-consuming: linear search required Hash Table linear list with hash data structure decreases directory search time collisions: situations where two file names hash to the same location

10 Allocation Methods Determines how disk blocks are allocated for files
Contiguous allocation Linked allocation Indexed allocation

11 Contiguous Allocation
Each file occupies a set of contiguous blocks on the disk Simple: Only starting location (block #) and length (number of blocks) are required Advantage: Random access Cons Dynamic storage allocation may become difficult Files may not be allowed to grow

12 Contiguous Allocation of Disk Space

13 Extent-Based Systems A modified contiguous allocation scheme
Extent-based file systems allocate disk blocks in extents An extent is a contiguous block of disks Initially, allocate contiguous disk space to a file. If the file grows, assign an extent. Keep the link to the extent in addition to the start and link in the previous slide.

14 Linked Allocation Each file is a linked list of disk blocks
Blocks may be scattered anywhere on the disk pointer block =

15 Linked Allocation

16 Linked Allocation Pros Cons Simple: Only needs the starting address
Easy to manage free space No waste of space Cons No random access -> slow down access

17 Indexed Allocation Brings all pointers together into the index block

18 Indexed Allocation Need index table -> Overhead
Random access without external fragmentation If max file size is fixed, the size of an index block can be determined Example Max file = 256K words, block size = 512 words -> Only 1 block is needed to store an index table Multi-level of indexes can be used, if necessary, to reduce the disk space consumption by index blocks If the file size is unlimited, link blocks of index table

19 Combined Scheme: UNIX (4K bytes per block)

20 Free-Space Management
Bit vector for n blocks 1 2 n-1 0  block[i] free 1  block[i] occupied bit[i] =  Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

21 Free Space Management Bit map requires extra space Example:
block size = 212 bytes disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes) Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space Grouping: Contiguous free blocks -> one group Counting: Keep the count rather than links

22 Linked Free Space List on Disk

23 Free Space Management Need to protect: Pointer to free list Bit map
Must be kept on disk Copy in memory and disk may differ Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk Solution: Set bit[i] = 1 in disk Allocate block[i] Set bit[i] = 1 in memory

24 Efficiency and Performance
Efficiency depends on: disk allocation and directory algorithms types of data kept in file’s directory entry Performance improvement techniques Disk cache: Separate section of main memory for frequently used blocks Free-behind and read-ahead: Techniques to optimize sequential access

25 Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O

26 Recovery Consistency checking compares data in directory structure with data blocks on disk, and tries to fix inconsistencies Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical) Recover lost file or disk by restoring data from backup

27 Log Structured File Systems
Log structured (or journaling) file systems record each update to the file system as a transaction Each transaction is written to a log A transaction is considered committed once it is written to the log But, the file system may not have been updated yet

28 Log Structured File Systems
The transactions in the log are asynchronously written to the file system After the file system in the disk is updated, the log record is removed from the log If the file system crashes, all remaing committed transactions in the log must still be performed Updates are NOT lost!

29 End of Chapter 11 Questions?

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