Da-Wei Chang CSIE.NCKU. Professor Hao-Ren Ke, National Chiao Tung University Professor Hsung-Pin Chang, National Chung Hsing University

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1 Chapter 11 Implementing File System Da-Wei Chang CSIE.NCKU Source: Professor Hao-Ren Ke, National Chiao Tung University Professor Hsung-Pin Chang, National Chung Hsing University

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

3 File-System Structure I/O transfers between memory and disk are performed in units of blocks File system: provide efficient and convenient access to disk Two design problems How the file system looks to the user File and its attributes, operations on a file Map the logical file system onto the physical disk

4 Layered File Systems file system core

5 Layered File System file name/id Application Programs write(data_file, item, size) Logical File System information of data_file File-Organization Module (FOM) 2144 th block Basic File System Given a file name/id, use directory structure to provide information needed by FOM Transform logical block address to physical block address Issue I/O command to device driver to physical block on the disk I/O Control Issue actual hardware commands Devices

6 Layered File System I/O control device drivers and interrupt handlers Transfer information between main memory and disk system Retrieve physical block XXX HW-specific commands Basic file system Manages the memory buffers/caches Issue generic I/O commands to device driver to read/write physical blocks on the disk

7 Layered File System (Cont.) File-organization module Know about files, their logical blocks, and physical blocks Translate logical blocks to physical blocks Logical blocks: 0 ~ N Free-space management: track free blocks in disk Block allocation: allocated free blocks when requested

8 Layered File System (Cont.) Logical file system manage metadata information Metadata: data used to manage file data directory structure File Control Blocks (FCB) a data structure contains information about a file each file has a FCB Ownership, permissions, and location of the file content Mentioned later In UNIX, an FCB is called an inode.

9 A Typical File Control Block

10 Layered File System (Cont.) Why layered file system? All the advantages of the layered approach Duplication of code is minimized for different file system standard Usually, the I/O control and the basic file system code can be used by multiple file system formats Common file system format: ext3 and ext4 (Extended File Systems 3 and 4), FAT, FAT32, NTFS

11 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery NFS

12 File-System Implementation A file system implementation has both on-disk data structures, and in-memory data structures

13 On Disk Structures - A Possible File System Layout Boot OS from the partition 2. Volume control block 3. Directory structure and file data 4. FCBs

14 On-Disk Structures Boot control block (per volume): information needed by the system to boot an OS from that partition Typically, first block in that volume If no OS empty UFS: called boot block; NTFS: called partition boot sector *UFS: Unix File System (also called Berkeley Fast File System or FFS) *A volume represents a physical partition here. Volume control block (per volume): volume details No. of blocks, size of the blocks, free-block count and freeblock pointers, free FCB count and FCB pointers UFS: called superblock; NTFS: called Master File Table

15 On-Disk Structures (Cont.) Directory structure (per FS): used to organize files UFS: includes file names and FCB numbers (inode #s) NTFS: within the Master File Table See the next slide A per-file File Control Block: details about a file File permissions, ownership, size, location of the data blocks.. UFS: called inode; NTFS: within the Master File Table

16 Directory Structure directory structure Soft Link? Hard link?

17 In-Memory Information Caching information to avoid retrieving the information every time from disk An in-memory mount table Contain information about each mounted volume The system-wide open-file table A copy of FCB of each open file Chapter 10 The per-process open-file table Chapter 10 Directory-structure cache Hold the information of recently accessed directories Buffer cache Hold the information of recently accessed file data

18 In-Memory File System Structures When opening a file, the OS Searches the directory structure for the given file name Caches parts of the directory structure Copies the FCB into the system-wide open-file table Creates an entry in the per-process open-file table File descriptor Return value of open() FP = fopen(filename, "r"); An index to the entry in the per-process table

19 In-Memory File System Structures o1 o2 file descriptor o5: return the file descriptor r1 o4 r2 r3 o3 file open (o1-o5) *o1-o2 are for pathname lookup file read (r1-r3)

20 Partitions and Mounting A disk can be sliced into multiple partitions Each partition can be Raw: contain no file system Example: swap space/area Cooked: contain a file system Root partition: contains OS (including the file system code) Mounted at boot time Other partitions are mounted at boot time or manually

21 Virtual File Systems How to integrated multiple types of file systems in a system? See the following two slides Sol: Virtual File Systems (VFS) File-system implementation consists of three major layers 1st layer: file-system interface: open(), write(), 2nd layer: virtual file system layer 3rd layer: implementation of the actual file systems

22 Multiple File Systems in a System EXT4 EXT2 FAT32 NTFS

23 Schematic View of Virtual File System (Cont.) Open, read, write

24 Virtual File Systems Two functions Separate file-system-generic operations from their implementation Several implementations for the VFS interface can coexist on the same machine See the following Linux example Provide a mechanism for uniquely representing a file throughput a network VFS maintain a file-representation structure, called a vnode, for a network-wide uniqueness

25 Example: VFS in Linux VFS in Linux uses four main objects inode object: FCB; information related to an individual file file object: represent an open file superblock object: represent an entire file system dentry object: represent an individual directory entry For each object, VFS defines a set of operations (interface) that must be implemented Each actual file systems must implement these operations

26 For superblock object, the operation interface defined by VFS EXT2 Implementation of the superblock operations

27 EXT2 Implementation of the Superblock Operations ext2_read_inode() {.. } ext2_write_inode() { }..

28 Operation Interface Defined by VFS for Inode Object

29 Operations Interface Defined by VFS for File Object

30 Virtual File Systems (Cont.) Thus, the file access API is issued to the VFS layer Rather than any specific type of file system Then, VFS calls the appropriate function from the operation table Depends on which file system is accessed E.g. read() system call vfs_read() ext2_read() Without knowing the details of how the file system is implemented

31 Outline File-System Structure File-System Implementation Directory Implementation Linear List Hash Table Allocation Methods Free-Space Management Efficiency and Performance Recovery NFS

32 Directory Implementation- Linear List Linear list of file names with pointer to the data blocks. Advantage Simple to program Disadvantage Time-consuming for searching Linear search to find a particular entry A solution cache the most recently used directory information

33 Linear List Implementation Suppose /pub/temp has three files: a.doc, 3.ppt, b.c a.doc 3.ppt b. c inode 134 inode 5 inode 677

34 Outline File-System Structure File-System Implementation Directory Implementation Linear List Hash Table Allocation Methods Free-Space Management Efficiency and Performance Recovery NFS Example: WAFL File System

35 Directory Implementation- Hash Table Hash Table linear list with hash data structure. (File name) => hash function => a pointer to the linear list Decreases directory search time Hash collisions? Two file names hash to the same location Each hash table entry is a linked list; solve the collision problem

36 Hash Table Implementation Suppose /pub/temp has three files: a.doc, 3.ppt, b.c f(a.doc)=3 f(b.c) =3 f(3.ppt)= ppt 5 a.doc b. c pointers are offsets in the directory file

37 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Contiguous Allocation Linked Allocation Indexed Allocation Free-Space Management Efficiency and Performance Recovery NFS Example: WAFL File System

38 Allocation Methods How to allocate space to files? so that disk space is utilized effectively and files can be accessed quickly An allocation method refers to how disk blocks are allocated for files. Contiguous allocation Extend-based system Linked allocation Indexed allocation

39 Contiguous Allocation Each file occupies a set of contiguous blocks on the disk Directory entry or FCB for each file Only starting location (block #) + length (number of blocks) See the next slide Advantages Fast -- minimal seek time and head movement Support both sequential and direct access Direct access to block i of a file that starts at block b, Access block b+i

40 Contiguous Allocation of Disk Space

41 (a) Contiguous allocation of disk space for 7 files (b) State of the disk after files D and F have been removed

42 Contiguous Allocation Finding space for a new file Similar to contiguous memory allocation problem Introduced in Chapter 8 Request n blocks from a list of free holes by first fit or best fit Disadvantages External fragmentation Solution: compaction (expensive) See the next following slide Files are difficult to grow Copy the content to a larger hole So, when creating a file, one may need to determine how much space is needed for a file

43 External Fragmentation A request of 650 KB. Total memory is big enough but cannot satisfy this request

44 Compaction (I) From Ka, Ha-Zen

45 Contiguous Allocation with Extent A modified contiguous allocation scheme to minimize the problem of contiguous allocations Extent-based file systems allocate disk blocks in extents An extent is a contiguous set of blocks Extents are allocated for file allocation A file consists of one or more extents. Integrate contiguous allocation and linked allocation (described later)

46 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Contiguous Allocation Linked Allocation Indexed Allocation Free-Space Management Efficiency and Performance Recovery NFS

47 Linked Allocation Each file is a linked list of disk blocks Blocks may be scattered anywhere on the disk Directory entry or FCB 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 block = pointer

48 Linked Allocation

49 Storing a file as a linked list of disk blocks

50 Linked Allocation (Cont.) Disadvantages Inefficient to support random/direct accesses Efficient only for sequential accesses To find i th block, must start at the beginning and follow the pointer Space required for the pointers Solutions: cluster scheme Reliability What if a pointer is lost (e.g. points to a free block or an used block of another file)?

51 Linked Allocation (Cont.) Solution for inefficient random/direct accesses FAT (describe in the next slide) Solution for spaces for pointers: cluster scheme Group blocks into clusters, and allocate clusters instead of single blocks use BIG blocks! Fewer disk head seeks Decrease the space overhead Pointers then uses a much smaller percentage Problem: internal fragmentation Solution for reliability Doubly linked list or store the filename and relative block number in each block More overhead for each file

52 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 Random access time is improved since FS doesn t need to traverse the data blocks to find the needed block Increased number of disk head seeks (for seq. accesses) One for FAT, one for data FAT may far from the data Improved by caching FAT Put all the pointers in FAT, instead of in the data blocks

53 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Contiguous Allocation Linked Allocation Indexed Allocation Free-Space Management Efficiency and Performance Recovery NFS

54 Indexed Allocation Bring all pointers together into the index block The i th entry points to the i th block of the file The directory entry or FCB contains the address of the index block Similar to paging scheme in memory management Page table = index block

55 Example of Indexed Allocation

56 Indexed Allocation (Cont.) Advantages Support direct access No external fragmentation Disadvantage Wasted space: space for index block Generally larger than the linked allocation If a file is smaller than a block Linked allocation: a pointer Index allocation: an entire index block

57 Indexed Allocation (Cont.) Challenge: how large the index block should be Large index block: waste space for small files Small index block: how to handle large files?

58 Indexed Allocation (Cont.) Mechanism for handling the index block Linked scheme: link together several index blocks Multilevel index: like multi-level paging A first-level index block points to a set of second-level index blocks Then second-level index blocks point to the file blocks Combined scheme: Direct blocks + indirect blocks + double indirect blocks + triple indirect blocks Used by BSD UNIX System

59 Linked Scheme directory file first index block jeep 19 Null index block next data data data data next data data data data next data data data data

60 Multilevel Index (Two-Level) first-level second-level

61 Combined Scheme: UNIX inode

62 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Bit vector (bit map) Linked list Grouping Counting Efficiency and Performance Recovery NFS

63 Free-Space Management Free-space management: keep track of free disk space bit vector (bit map) linked list grouping counting

64 Bit Vector Simple to find the first free block, or n consecutive free blocks Example: 2~5, 8~13, 17~18, 25~27 blocks are free, others are allocated Bit vector =

65 Bit Vector Inefficient unless the entire vector is kept in main memory So only for small disks? Example: 1.3 GB disks need a bit map of over 332 KB block size = 2 9 bytes (512 bytes) disk size = 1.3 x 2 30 bytes (1.3 gigabyte) n = 1.3 x 2 30 /2 9 = 1.3 x 2 21 bits = (or K bytes)

66 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Bit vector (bit map) Linked list Grouping Counting Efficiency and Performance Recovery NFS

67 Linked List Link together all free blocks Keep a pointer to the first free block See the next slide Advantages No waste of space Disadvantages Cannot get contiguous space easily Not easy to traverse the list Fortunately, OS needs one free block at a time Just find the first free block, no traversal FAT incorporate the linked list mechanism Not always valid for modern OS

68 Linked Free Space List on Disk

69 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Bit vector (bit map) Linked list Grouping Counting Efficiency and Performance Recovery NFS

70 Grouping And Counting Grouping: Store the address of n free blocks in the first free block. The n th block contains the addresses of another n free blocks and so on Good: find a large number of free blocks quickly Counting: Keep the address of the first free blocks and the number n of free contiguous blocks Since several contiguous blocks may be allocated or freed at the same time

71 Example Of Free-Space Management Bit Vector Grouping (n = 3) Block 2 stores (3, 4, 5) Block 5 stores (8, 9, 10) Block 10 stores (11, 12, 13) Block 13 stores (17, 28, 25) Block 25 stores (26, 27, -1) Counting

72 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery NFS

73 Efficiency Efficiency: disk space utilization Methods Minimize the metadata, internal fragmentation, external fragmentation Example: in FAT Each entry was 12 bits, pointing to 8-KB cluster Efficient but support only 32 MB disk drive (2^12 x 2^ 13 B = 2 ^25 B = 32 MB) Thus, each FAT entry was changed to 16 bits, and later 32 bits

74 Performance Performance (MB/s or IOPS) On-board cache local memory in disk controller to store entire tracks at a time Buffer cache separate section of main memory for frequently used blocks Mentioned later Synchronous writes v.s. asynchronous writes Mentioned later Free-behind and read-ahead techniques to optimize sequential access Mentioned later

75 Various Caching Locations

76 Performance Buffer Cache and Page Cache Buffer cache: frequently accessed blocks are kept in memory A page cache caches pages rather than disk blocks by virtual memory techniques More efficient than caching through blocks Since accesses throughput virtual memory is more efficient than file systems Due to layering architecture in file systems

77 I/O Without a Unified Buffer Cache

78 I/O Without a Unified Buffer Cache Problem: double caching See the previous figure Memory-mapped I/O uses a page cache Routine I/O through the file system uses the buffer cache Data may be cached in both buffer cache and page cache Buffer cache: for using ordinary file operations Page cache: for using memory-mapped file Solution: unified buffer cache

79 Unified Buffer Cache Unified buffer cache Uses the same cache to cache both memorymapped pages and ordinary file system I/O

80 I/O Using a Unified Buffer Cache

81 Synchronous and Asynchronous Writes Synchronous writes Writes are not buffered Caller must wait for the data to reach the disk drive Metadata are usually synchronous writes Directory entries, FCB Asynchronous writes Writes are stored in the cache and return to the caller

82 Free-Behind and Read-Ahead Usually, replacement algorithm of page cache: LRU However, LRU is not suitable for sequential access Free-behind Remove a page from the buffer as soon as the next page is requested The previous pages are not likely to be used again Read-ahead A requested page and several subsequent pages are read and cached These pages are likely to be requested soon Both are applied to sequential access

83 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery Consistency Checking Log-Structured File Systems Other Solutions Backup and Restore NFS

84 Recovery Some directory information is kept in main memory If a computer crash, information in memory are lost Cache, buffer contents, and the current I/O operation The file system may be in inconsistent state

85 Consistency Checker Consistency checker check metadata and tries to fix inconsistencies e.g. an inode points to a free data block Examples UNIX: fsck Windows: chkdsk

86 Log Structured File Systems Log structured (or journaling) file systems record each metadata update to the file system as a transaction All metadata changes (i.e., 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 Then, the changes are made on the actual file system If the file system crashes the committed transactions in the log is replayed to bring the FS to a consistent state

87 Backup and Restore 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 Incremental backup v.s. full backup

88 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery NFS

89 The Sun Network File System (NFS) An implementation and a specification of a software system for accessing remote files across LANs (or WANs) Goal: allows sharing among file systems on different machines in a transparent manner Based on client-server model

90 NFS (Cont.) Steps A remote directory is mounted over a local directory Specification of the remote directory for the mount operation is nontransparent The host name of the remote directory has to be provided Next two Figures But, after than, files in the remote directory can be accessed in a transparent manner

91 Three Independent File Systems

92 Mounting Results mount S1:/user/shared over U:/user/local

93 NFS (Cont.) NFS is designed to operate in a heterogeneous environment Different machines, operating systems, and network architectures This independence is achieved through the RPC primitives built on top of an External Data Representation (XDR) See the next slide The NFS specification defines two services The services provided by a mount mechanism : the mount protocol The actual remote-file-access services : the NFS protocol

94 Schematic View of NFS Architecture

95 The Mount Protocol Implement the mount operations Mount request is mapped to corresponding RPC and forwarded to mount server running on server machine Then, 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 The mount operation changes only the user s view and does not affect the server side

96 NFS Protocol Provides a set of RPCs 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 No open() and close() Each request has to provide a full set of arguments Include a unique file identifier and offset

97 Three Major Layers of NFS Architecture See the next slide Layer 1: UNIX file-system interface open(), read(), write(), and close() calls Layer 2: Virtual File System (VFS) Distinguishes local files from remote ones If remote, calls the NFS protocol for remote requests Layer 3: NFS service layer Implements the NFS protocol

98 Schematic View of NFS Architecture

99 NFS 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 performance File block and file attributes are fetched by RPCs and are cached locally

100 NFS Remote Operations (Cont.) Two Caches File-attribute cache the inode information File-blocks cache the data blocks Read-ahead and delay-write are used

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