chapter 11: implementing file systems
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Chapter 11: Chapter 11: Implementing File SystemsImplementing File Systems
11.2 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Chapter 11: Implementing File SystemsChapter 11: Implementing File Systems
File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Efficiency and Performance
Recovery
Log-Structured File Systems
11.9 NFS (skip)
11.10 Example: WAFL File System (skip)
11.3 Silberschatz, Galvin and Gagne ©2005Operating System Principles
ObjectivesObjectives
To describe the details of implementing local file systems and directory structures
To describe the implementation of remote file systems (11.9, skip)
To discuss block allocation and free-block algorithms and trade-offs
11.4 Silberschatz, Galvin and Gagne ©2005Operating System Principles
11.1 File-System Structure11.1 File-System Structure
Disk characteristics for storing multiple files Can be rewritten in place Can access directly any block of information it contains
File structure Logical storage unit Collection of related information
File system resides on secondary storage (disks) Allow the data in disk to be stored, located, and retrieved easily
How the file system should look to the user How to map the logical file system to the physical secondary
storage devices
File system organized into layers File control block (FCB) – storage structure consisting
of information about a file
11.5 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Layered File SystemLayered File System
Device driver
Translates logical block addresses to physical block addresses
Manages metadata information
System calls like create( ), open( ), close( )
11.6 Silberschatz, Galvin and Gagne ©2005Operating System Principles
11.2 File-System Implementation11.2 File-System Implementation
On-disk and in-memory structures are used to implement a file system
On disk A boot control block A volume control block A per-file FCB: file permissions, ownership, size, and location
of the data blocks In UNIX File System, it is called inode In Windows NTFS, it is stored as a record in master file table
A directory structure In Unix File System, this include file names and associated inode
numbers In NTFS, it is stored in the master file table
11.7 Silberschatz, Galvin and Gagne ©2005Operating System Principles
11.2 File-System Implementation11.2 File-System Implementation
In-memory information is used for file-system management and performance improvement via caching In-memory mount table
In-memory directory-structure cache
System-wide open-file table A copy of the FCB for each open file
Per-process open-file table A pointer to the appropriate entry in system-wide open-file table
In Unix, it is called a file descriptor
In Windows, it is called a file handler
11.8 Silberschatz, Galvin and Gagne ©2005Operating System Principles
A Typical File Control BlockA Typical File Control Block
11.9 Silberschatz, Galvin and Gagne ©2005Operating System Principles
In-Memory File System StructuresIn-Memory File System Structures
The following figure illustrates the necessary file system structures provided by the operating systems.
Figure 11-3(a) refers to opening a file.
11.10 Silberschatz, Galvin and Gagne ©2005Operating System Principles
In-Memory File System StructuresIn-Memory File System Structures
Figure 11-3(b) refers to reading a file.
11.11 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Partitions and MountingPartitions and Mounting A disk can be sliced into multiple partitions. A volume can
span multiple partitions on multiple disks (RAID, Section 12.7)
Raw disk: no file system. Used in Unix swap space, and database management systems
Boot information has its own format, and is usually a sequential series of blocks, loaded as an image into memory Allow dual-booted for installing multiple OS
The root partition, containing the OS kernel and other system files, is mounted at boot time Other volumes can be automatically mounted at boot time or
manually mounted later OS maintains a mount table for mounted file systems Skip
11.2.3
11.12 Silberschatz, Galvin and Gagne ©2005Operating System Principles
11.3 Directory Implementation11.3 Directory Implementation
Linear list of file names with pointer to the data blocks. simple to program but time-consuming to execute To create a new file, directory must be searched to be sure
that no existing file has the same name. To delete a file, we search the directory for the named file, then release the space allocated to it
To reuse the directory entry, several options Mark the entry as unused by
– Assigning it s special name– Or with a used-unused bit
Attach it to a list of free directory entries Copy the last entry in the directory into the freed location and
decrease the length of the directory
Disadvantage: finding a file requires a linear search Make a list sorted would complicate the creating and deleting of
files
11.13 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Directory ImplementationDirectory Implementation
Hash Table – linear list stores the directory entries with a hash table The hash table takes a value computed from the file name and
returns a pointer to the file name in the linear list decreases directory search time Some provisions must be made for collisions – situations where
two file names hash to the same location Difficulties:
fixed size (because it is a table) The dependence of the hash function on that size
Alternatively, a chained-overflow hash table can be used instead each hash entry is a linked list instead of an individual value Collisions resolved by adding the new entry to the linked list
11.14 Silberschatz, Galvin and Gagne ©2005Operating System Principles
11.4 Allocation Methods11.4 Allocation Methods
An allocation method refers to how disk blocks are allocated for files. How to allocate space to these files so that disk space is
utilized effectively and files can be accessed quickly
Three major methods Contiguous allocation
Linked allocation
Indexed allocation
11.15 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Contiguous AllocationContiguous Allocation
Each file occupies a set of contiguous blocks on the disk Simple – only starting location (block #) and length (number of
blocks) are required
Random access (next page)
Problems Dynamic storage-allocation problem
First-fit, best-fit, worst-fit
Repacking off-line or on-line
Determining how much space is needed for a file when it is created. If we allocate too little space to a file, it cannot be extended
Pre-allocation may be inefficient
11.16 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Contiguous AllocationContiguous Allocation
Mapping from logical to physical
Logical Address/512
Q (Quotient)
R (Remainder)
Block to be accessed = Q + starting addressDisplacement into block = R
11.17 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Contiguous Allocation of Disk SpaceContiguous Allocation of Disk Space
11.18 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Extent-Based SystemsExtent-Based Systems
Many newer file systems (I.e. Veritas File System) use this modified contiguous allocation scheme
Extent-based file systems allocate disk blocks in extents A contiguous chunk of space is allocated initially
If that amount is not large enough later, another chunk of contiguous space, called extent, is added
A file consists of one or more extents. The location of a file’s blocks is recorded as a location and a
block count, plus a link to the first block of the next extent
11.19 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Linked AllocationLinked Allocation
Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.
The directory contains for each file a pointer to the first and last blocks of the file
Pointer to the next block
Contents of a block
11.20 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Linked Allocation (Cont.)Linked Allocation (Cont.)
Simple – need only starting address
Free-space management system – no waste of space
No random access
Mapping
Block to be accessed is the Q-th block in the linked chain of blocks representing the file.Displacement into block = R + 1
File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2.
Logical Address/511Q (Quotient)
R (Remainder)
11.21 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Linked AllocationLinked Allocation
Disadvantages:1. Can be used effectively only for sequential-access files2. The space required for the
pointers. Solution: collect blocks into
clusters, and allocate clusters rather than blocks. Cost is increased internal fragmentation.
3. Reliability: disaster if pointers were lost or damaged.
Solution: doubly linked lists or store file name and relative block number in each block.
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11.22 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Linked AllocationLinked Allocation
Linear list of file names with pointer to the data blocks simple to program
time-consuming to execute
Hash Table – linear list with hash data structure decreases directory search time
collisions – situations where two file names hash to the same location
fixed size
11.23 Silberschatz, Galvin and Gagne ©2005Operating System Principles
File-Allocation Table (MS-DOS and OS/2)File-Allocation Table (MS-DOS and OS/2)
end-of-file
Unused block: a 0 table value
Allocating a new block to a file: Finding the first 0-valued table
entry and replacing the previous end-of-file value with the address of the new block. The 0 table entry is then replaced by the
end-of-file value.
11.24 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Indexed AllocationIndexed Allocation
Brings all pointers together into the index block
Each file has its own index block
Logical view.
index table
11.25 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Example of Indexed AllocationExample of Indexed Allocation
11.26 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Indexed AllocationIndexed Allocation Need index table Random access Dynamic access without external fragmentation, but
have overhead of index block With only 1 block for index table and a block size of
512 words, mapping from logical to physical in a file of maximum size of 256K (=0.5K * 512) words.
Logical Address/512Q (Quotient)
R (Remainder)
Q = displacement into index tableR = displacement into block
11.27 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Indexed AllocationIndexed Allocation If the index block is too small, it will not be able to hole
enough pointers for a large file. Mechanisms to handle this issue: Linked scheme
The last word in the index block is nil (for a small file) or is a pointer to another index block (for a large file)
Multilevel index Use first-level index block to point to a set of second-level index
blocks, which point to the file blocks
Combined scheme Example: In Unix File System, for the 15 pointers of the index
block in the file’s inode– The first 12 point to data of the file
– The next three pointers point to (single, double, triple) indirect blocks
11.28 Silberschatz, Galvin and Gagne ©2005Operating System Principles
Combined Scheme: UNIX (4K bytes per block)Combined Scheme: UNIX (4K bytes per block)
11.29 Silberschatz, Galvin and Gagne ©2005Operating System Principles
PerformancePerformance
Before selecting an allocation method, we need to know how the system would be used Contiguous allocation requires only one access to get a disk block.
Linked allocation is only good for sequential access. Some system supports both, but require the declaration of the type of
access in file creation.
Keeping index block in memory requires considerable space. The performance of indexed allocation depends on the index structure (how many level), on the size of the file, and on the position of the block desired. Some system uses contiguous allocation for small files and
automatically switching to an index allocation if the file grows large
Many other optimizations are in use It is reasonable to add (hundreds of) thousands of instructions to save
a few disk-head movements
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