File-System File-System ImplementationImplementation

1

OutlineOutlineFile-System StructureFile-System ImplementationDirectory ImplementationAllocation MethodsFree-Space ManagementEfficiency and PerformanceRecoveryLog-Structured File SystemNFS

2

File-System StructureFile-System Structure

3

IntroductionIntroductionFile structure

◦ Logical storage unit◦ Collection of related information

File system resides on secondary storage (disks)

File system organized into layersFile control block – storage structure

consisting of information about a file◦ Ownership, permissions, and location of

the file contentI/O transfers between memory and disk

are performed in units of blocks (one more more sectors)

4

Layered File SystemLayered File System

5

Layered File System Layered File System (Cont.)(Cont.)I/O control – device drivers and

interrupt handlers◦Transfer information between main

memory and disk system◦Retrieve block 123 HW-specific

instructionsBasic file system

◦ Issue generic commands to device driver to read and write physical blocks on the disk

◦Physical block: drive 1, cylinder 73, track 2, sector 10

6

Layered File System Layered File System (Cont.)(Cont.)File-organization module

◦ Know about files, their logical blocks, and physical blocks

◦ Translate logical blocks to physical blocks (similar to VM) Logical blocks: 0 – N

◦ Free-space manager◦ Blocks allocation

Logical file system – manage metadata information◦ Metadata: file-system structure, excluding the

actual file contents◦ Manage the directory structure via file control

blocks (FCB)

7

Layered File System Layered File System (Cont.)(Cont.)Why Layered file system?

◦All the advantages of the layered approach

◦File system standard: UFS, FAT FAT32, NTFS…

◦Duplication of code is minimized for different file system standard

◦Usually I/O control and the basic file system code can be used by multiple file system formats.

8

A Typical FCBA Typical FCB

9

File System File System ImplementationImplementation

10

On-Disk StructuresOn-Disk StructuresBoot control block: information needed by

the system to boot an OS from that partition◦ UFS: boot block; NTFS: partition boot sector

Partition control block: partition details◦ No. of blocks, size of the blocks, free-block

count and free-block pointers, free FCB count and FCB pointers

◦ UFS: superblock; NTFS: Master File TableA directory structure is used to organize

the filesFile control block: many of the file’s details

◦ File permissions, ownership, size, location of the data blocks

◦ UFS: inode; NTFS: within the Master File Table11

In-Memory StructuresIn-Memory StructuresAn in-memory partition table

containing information about each mounted partition

An in-memory directory structure that holds the directory information of recently accessed directories

12

Caching information so that no need to retrieve the information every time from the disk

In-Memory File-System In-Memory File-System StructuresStructures

13

File Open

File Read

Virtual File SystemsVirtual File SystemsVirtual File Systems (VFS) provide an

object-oriented way of implementing file systems

VFS separates file-system-generic operations from their implementation by defining a clean VFS interface

VFS allows the same system call interface (the API) to be used for different types of file systems

VFS is based on a file-representation structure, called a vnode, that contains a numerical designator for a network-wide unique file

The API is to the VFS interface, rather than any specific type of file system

14

Schematic View of Virtual Schematic View of Virtual File SystemFile System

15

Open, read, write…

Directory ImplementationDirectory ImplementationLinear list of file names with pointer to

the data blocks◦ Simple to program◦ Time-consuming to execute – linear search

to find a particular entry Cache and sorted list may help

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 and the dependence of the hash

function on that size16

Allocation MethodsAllocation MethodsHow to allocate space to files so that disk space is utilized effectively and files can be accessed quickly

17

Contiguous AllocationContiguous AllocationA file occupies a set of contiguous blocks on

diskOnly starting block (block #) and length

(number of blocks) are required in the directory entry (FCB)

Fast -- Minimal seek time and head movementRandom access any block within the fileSimilar to dynamic storage-allocation problem

◦ External fragmentation – may need compactionFiles are difficult to grow

◦ Find a larger hole and copy the file to the new space

18

Contiguous Allocation Contiguous Allocation (Cont.)(Cont.)

19

Extent-Based SystemsExtent-Based SystemsMany newer file systems (I.e.

Veritas File System) use a modified contiguous allocation scheme

Extent-based file systems allocate disk blocks in extents

An extent is a contiguous block of disks. Extents are allocated for file allocation. A file consists of one or more extents.

Integrate contiguous allocation and linked allocation (see later)

20

Linked AllocationLinked AllocationEach file is a linked list of disk blocks

◦ Blocks may be scattered anywhere on the disk

◦ Directory contains a pointer to the first and last blocks

◦ Each block contains a pointer to the next block

Advantages◦ No external fragmentation◦ Easy to grow – Any free block is OK

Disadvantages◦ Effectively for only sequential-access file◦ Space required for the pointers◦ Reliability – What if the pointers are lost

21

Linked Allocation (Cont.)Linked Allocation (Cont.)

22

pointer

data

block =

Linked Allocation (Cont.)Linked Allocation (Cont.)Solution for spaces for pointers

◦Collect blocks into clusters, and allocate the clusters than blocks ( 每次 Allocate一個 Cluster, 而非一個 Block)

◦Fewer disk head seeks and decreases the space needed for block allocation and free-list management

◦Internal fragmentationSolution for reliability

◦Double linked list or store the filename and relative block number in each block More overhead for each file

23

Linked Allocation (Cont.)Linked Allocation (Cont.)

FAT (File Allocation Table)◦ OS/2, MS-DOS◦ The table has one entry for

each disk block and is indexed by block number Similar to the linked list Contain the block number

of the next block in the file◦ Significant number of disk

head seeks One for FAT, one for data Improved by caching FAT Random access time is

improved

24

把 Pointer集中放置於 FAT，而不是跟 Data Block放一起

Indexed AllocationIndexed AllocationBring all pointers together into

the index block◦An array of disk-block addresses◦The ith entry points to the ith block

of the file◦The directory contains the address of

the index block◦Similar to the paging scheme for

memory management

25

Example of Indexed Example of Indexed AllocationAllocation

26

Indexed Allocation (Cont.)Indexed Allocation (Cont.)Advantage

◦ Support random access◦ Dynamic access without external

fragmentation◦ No size-declaration problem◦ But have overhead of index block. Need

index tableDisadvantage

◦ Wasted space: Worse than the linked allocation for small files

How large the index block should be◦ Large index block: waste space for small

files◦ Small index block: how to handle large files

27

Indexed Allocation (Cont.)Indexed Allocation (Cont.)Mechanism for handling the index

block◦Linked scheme: Link together several

index blocks◦Multilevel index: like multi-level paging

With 4096-byte blocks, we could store 1024 4-byte pointers in an index block. Two levels of indexes allows 1,048,576 data blocks, which allow a file of up to 4 gigabytes

◦Combined scheme: For example BSD UNIX System

28

Indexed Allocation Indexed Allocation –– Multilevel Index (Cont.)Multilevel Index (Cont.)

29

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

30

The UNIX inode

How large can a file be, if each pointer in the index blocks is 4-bytes?

Free Space ManagementFree Space Management

31

Bit VectorBit Vector Simple and efficient to

find the first free block, or consecutive free blocks◦ By bit-manipulation

Requires extra space◦ block size = 212 bytes◦ disk size = 230 bytes◦ n = 230/212 = 218 bits (or

32K bytes) Efficient only when the

entire vector is kept in main memory◦ Write back to the disk

occasionally for recovery needs

32

001111001111100011000011100…

…0 1 2 n-1

bit[i] = 0 block[i] free

1 block[i] occupied

Question: What’s the block # of the fist free block?

Linked ListLinked ListLink together all free blocksKeep a pointer to the first free block in

a special location on the disk and caching it in memory

Cannot get contiguous space easilyNo waste of spaceNot efficient: have to traverse the disk

for free spaces◦ Usually, OS needs one free block at a

timeFAT incorporate the linked list

mechanism33

Grouping And CountingGrouping And CountingGrouping: store the address of n

free blocks in the first free block. The first n-1 are actually free. The final block contains the addresses of another n free blocks…

Counting: Each entry has a disk address and a count◦Several contiguous blocks may be

allocated or freed simultaneously34

Example Of Free-Space Example Of Free-Space ManagementManagement

35

Bit Vector 11000011000000111001111110001111

Counting 2 4 8 6 17 2 25 3

Grouping Block 2 3, 4, 5 Block 5 8, 9, 10 Block 10 11, 12, 13 Block 13 17, 28, 25 Block 25 26, 27

Efficiency and Efficiency and PerformancePerformance

36

Efficiency and Efficiency and PerformancePerformanceEfficiency dependent on

◦ Disk allocation and directory algorithms◦ Types of data kept in file’s directory entry

Performance◦ On-board cache – local memory in disk

controller to store entire tracks at a time◦ Disk cache – separate section of main

memory for frequently used blocks (LRU is a reasonable algorithm for block replacement)

◦ Free-behind and read-ahead – techniques to optimize sequential access (optimize the disk cache’s block replacement algorithm)

◦ Improve PC performance by dedicating section of memory as virtual disk, or RAM disk.

37

Various Disk-Caching Various Disk-Caching LocationsLocations

38

Page CachePage CacheNon-unified buffer cache

◦A page cache caches pages rather than disk blocks using virtual memory techniques

◦Memory-mapped I/O uses a page cache◦Routine I/O through the file system uses

the buffer (disk) cacheUnified Buffer Cache

◦ A unified buffer cache uses the same buffer cache to cache both memory-mapped pages and ordinary file system I/O

39

I/O Without/With A Unified I/O Without/With A Unified Buffer CacheBuffer Cache

40

RecoveryRecoveryConsistency checker – 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)

Recover lost file or disk by restoring data from backup

41

Log Structured File Log Structured File SystemsSystemsLog structured (or journaling) file systems

record each update to the file system as a transaction

All transactions are written to a log. A transaction is considered committed once it is written to the log

However, the file system may not yet be updated

The transactions in the log are asynchronously written to the file system. When the file system is modified, the transaction is removed from the log

If the file system crashes, all remaining transactions in the log must still be performed

42

NFSNFS

43

The Sun Network File System The Sun Network File System (NFS)(NFS)An implementation and a specification

of a software system for accessing remote files across LANs (or WANs)

The implementation is part of the Solaris and SunOS operating systems running on Sun workstations using an unreliable datagram protocol (UDP/IP protocol) and Ethernet

Interconnected workstations viewed as a set of independent machines with independent file systems, which allows sharing among these file systems in a transparent manner

44

NFS (Cont.)NFS (Cont.)A remote directory is mounted over a local

file system directory. The mounted directory looks like an integral subtree of the local file system, replacing the subtree descending from the local directory

Specification of the remote directory for the mount operation is nontransparent; the host name of the remote directory has to be provided. Files in the remote directory can then be accessed in a transparent manner

Subject to access-rights accreditation, potentially any file system (or directory within a file system), can be mounted remotely on top of any local directory

45

NFS (Cont.)NFS (Cont.)NFS is designed to operate in a

heterogeneous environment of different machines, operating systems, and network architectures; the NFS specifications independent of these media.

This independence is achieved through the use of RPC primitives built on top of an External Data Representation (XDR) protocol used between two implementation independent interfaces.

The NFS specification distinguishes between the services provided by a mount mechanism and the actual remote file-access services.

46

Three Independent FSThree Independent FS

47

Mounting in NFSMounting in NFS

48mount 1S:/usr/shared/dir1 /usr/local

mount S2:/usr/dir2 /usr/local/dir1

NFS Mount ProtocolNFS Mount Protocol Establishes initial logical connection between server and

client Mount operation includes name of remote directory to be

mounted and name of server machine storing it. Mount request is mapped to corresponding RPC and

forwarded to mount server running on server machine Export list – specifies local file systems that server

exports for mounting, along with names of machines that are permitted to mount them.

Following a mount request that conforms to its export list, the server returns a file handle—a key for further accesses.

File handle – a file-system identifier, and an inode number to identify the mounted directory within the exported file system.

The mount operation changes only the user’s view and does not affect the server side.

49

NFS ProtocolNFS Protocol Provides a set of remote procedure calls for remote file

operations. The procedures support the following operations:◦ Searching for a file within a directory◦ Reading a set of directory entries◦ Manipulating links and directories◦ Accessing file attributes◦ Reading and writing files

NFS servers are stateless; each request has to provide a full set of arguments

Modified data must be committed to the server’s disk before results are returned to the client (lose advantages of caching)

The NFS protocol does not provide concurrency-control mechanisms

50

Three Major Layers of NFS Three Major Layers of NFS ArchitectureArchitectureUNIX file-system interface (based on the

open, read, write, and close calls, and file descriptors)

Virtual File System (VFS) layer – distinguishes local files from remote ones, and local files are further distinguished according to their file-system types◦ The VFS activates file-system-specific operations

to handle local requests according to their file-system types

◦ Calls the NFS protocol procedures for remote requests

NFS service layer – bottom layer of the architecture; implements the NFS protocol.

51

Schematic View of NFS Schematic View of NFS ArchitectureArchitecture

52

NFS Path-Name NFS Path-Name TranslationTranslationPerformed by breaking the path

into component names and performing a separate NFS lookup call for every pair of component name and directory vnode.

To make lookup faster, a directory name lookup cache on the client’s side holds the vnodes for remote directory names.

53

NFS Remote OperationsNFS Remote Operations Nearly one-to-one correspondence between regular

UNIX system calls and the NFS protocol RPCs (except opening and closing files)

NFS adheres to the remote-service paradigm, but employs buffering and caching techniques for the sake of performance

File-blocks cache – when a file is opened, the kernel checks with the remote server whether to fetch or revalidate the cached attributes. Cached file blocks are used only if the corresponding cached attributes are up to date.

File-attribute cache – the attribute cache is updated whenever new attributes arrive from the server.

Clients do not free delayed-write blocks until the server confirms that the data have been written to disk.

54