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Nội dung Text: Bài giảng Hệ điều hành nâng cao - Chapter 11: File System Implementation
- Chapter 11: File System Implementation
Operating System Concepts– 8th 8th Edition
Operating System Concepts – Edition 11.1 Silberschatz, Galvin and Gagne ©2009
- Chapter 11: File System Implementation
s File-System Structure
s File-System Implementation
s Directory Implementation
s Allocation Methods
s Free-Space Management
s Efficiency and Performance
s Recovery
s NFS
s Example: WAFL File System
Operating System Concepts – 8th Edition 11.2 Silberschatz, Galvin and Gagne ©2009
- Objectives
s To describe the details of implementing local file systems and directory structures
s To describe the implementation of remote file systems
s To discuss block allocation and free-block algorithms and trade-offs
Operating System Concepts – 8th Edition 11.3 Silberschatz, Galvin and Gagne ©2009
- File-System Structure
s File structure
q Logical storage unit
q Collection of related information
s File system resides on secondary storage (disks)
q Provided user interface to storage, mapping logical to physical
q Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily
s Disk provides in-place rewrite and random access
q I/O transfers performed in blocks of sectors (usually 512 bytes)
s File control block – storage structure consisting of information about a file
s Device driver controls the physical device
s File system organized into layers
Operating System Concepts – 8th Edition 11.4 Silberschatz, Galvin and Gagne ©2009
- Layered File System
Operating System Concepts – 8th Edition 11.5 Silberschatz, Galvin and Gagne ©2009
- File System Layers
s Device drivers manage I/O devices at the I/O control layer
q Given commands like “read drive1, cylinder 72, track 2, sector 10, into memory location 1060” outputs low-
level hardware specific commands to hardware controller
s Basic file system given command like “retrieve block 123” translates to device driver
s Also manages memory buffers and caches (allocation, freeing, replacement)
s Buffers hold data in transit
s Caches hold frequently used data
s File organization module understands files, logical address, and physical blocks
s Translates logical block # to physical block #
s Manages free space, disk allocation
Operating System Concepts – 8th Edition 11.6 Silberschatz, Galvin and Gagne ©2009
- File System Layers (Cont.)
s Logical file system manages metadata information
s Translates file name into file number, file handle, location by maintaining file control blocks (inodes in Unix)
s Directory management
s Protection
s Layering useful for reducing complexity and redundancy, but adds overhead and can decrease performance
s Logical layers can be implemented by any coding method according to OS designer
s Many file systems, sometimes many within an operating system
s Each with its own format (CD-ROM is ISO 9660; Unix has UFS, FFS; Windows has FAT, FAT32, NTFS as well
as floppy, CD, DVD Blu-ray, Linux has more than 40 types, with extended file system ext2 and ext3 leading;
plus distributed file systems, etc)
s New ones still arriving – ZFS, GoogleFS, Oracle ASM, FUSE
Operating System Concepts – 8th Edition 11.7 Silberschatz, Galvin and Gagne ©2009
- File-System Implementation
s We have system calls at the API level, but how do we implement their functions?
q On-disk and in-memory structures
s Boot control block contains info needed by system to boot OS from that volume
q Needed if volume contains OS, usually first block of volume
s Volume control block (superblock, master file table) contains volume details
q Total # of blocks, # of free blocks, block size, free block pointers or array
s Directory structure organizes the files
q Names and inode numbers, master file table
s Per-file File Control Block (FCB) contains many details about the file
q Inode number, permissions, size, dates
q NFTS stores into in master file table using relational DB structures
Operating System Concepts – 8th Edition 11.8 Silberschatz, Galvin and Gagne ©2009
- A Typical File Control Block
Operating System Concepts – 8th Edition 11.9 Silberschatz, Galvin and Gagne ©2009
- In-Memory File System Structures
s Mount table storing file system mounts, mount points, file system types
s The following figure illustrates the necessary file system structures provided by the operating systems
s Figure 12-3(a) refers to opening a file
s Figure 12-3(b) refers to reading a file
s Plus buffers hold data blocks from secondary storage
s Open returns a file handle for subsequent use
s Data from read eventually copied to specified user process memory address
Operating System Concepts – 8th Edition 11.10 Silberschatz, Galvin and Gagne ©2009
- In-Memory File System Structures
Operating System Concepts – 8th Edition 11.11 Silberschatz, Galvin and Gagne ©2009
- Partitions and Mounting
s Partition can be a volume containing a file system (“cooked”) or raw – just a sequence of blocks with no file system
s Boot block can point to boot volume or boot loader set of blocks that contain enough code to know how to load the
kernel from the file system
q Or a boot management program for multi-os booting
s Root partition contains the OS, other partitions can hold other Oses, other file systems, or be raw
q Mounted at boot time
q Other partitions can mount automatically or manually
s At mount time, file system consistency checked
q Is all metadata correct?
4 If not, fix it, try again
4 If yes, add to mount table, allow access
Operating System Concepts – 8th Edition 11.12 Silberschatz, Galvin and Gagne ©2009
- Virtual File Systems
s Virtual File Systems (VFS) on Unix provide an object-oriented way of implementing file systems
s VFS allows the same system call interface (the API) to be used for different types of file systems
q Separates file-system generic operations from implementation details
q Implementation can be one of many file systems types, or network file system
4 Implements vnodes which hold inodes or network file details
q Then dispatches operation to appropriate file system implementation routines
s The API is to the VFS interface, rather than any specific type of file system
Operating System Concepts – 8th Edition 11.13 Silberschatz, Galvin and Gagne ©2009
- Schematic View of Virtual File System
Operating System Concepts – 8th Edition 11.14 Silberschatz, Galvin and Gagne ©2009
- Virtual File System Implementation
s For example, Linux has four object types:
q inode, file, superblock, dentry
s VFS defines set of operations on the objects that must be implemented
q Every object has a pointer to a function table
4 Function table has addresses of routines to implement that function on that object
Operating System Concepts – 8th Edition 11.15 Silberschatz, Galvin and Gagne ©2009
- Directory Implementation
s Linear list of file names with pointer to the data blocks
q Simple to program
q Time-consuming to execute
4 Linear search time
4 Could keep ordered alphabetically via linked list or use B+ tree
s Hash Table – linear list with hash data structure
q Decreases directory search time
q Collisions – situations where two file names hash to the same location
q Only good if entries are fixed size, or use chained-overflow method
Operating System Concepts – 8th Edition 11.16 Silberschatz, Galvin and Gagne ©2009
- Allocation Methods - Contiguous
s An allocation method refers to how disk blocks are allocated for files:
s Contiguous allocation – each file occupies set of contiguous blocks
q Best performance in most cases
q Simple – only starting location (block #) and length (number of blocks) are required
q Problems include finding space for file, knowing file size, external fragmentation, need for
compaction off-line (downtime) or on-line
Operating System Concepts – 8th Edition 11.17 Silberschatz, Galvin and Gagne ©2009
- Contiguous Allocation
s Mapping from logical to physical
Q
LA/512
R
Block to be accessed = Q + starting address
Displacement into block = R
Operating System Concepts – 8th Edition 11.18 Silberschatz, Galvin and Gagne ©2009
- Contiguous Allocation of Disk Space
Operating System Concepts – 8th Edition 11.19 Silberschatz, Galvin and Gagne ©2009
- Extent-Based Systems
s Many newer file systems (i.e., Veritas File System) use a modified contiguous allocation scheme
s Extent-based file systems allocate disk blocks in extents
s An extent is a contiguous block of disks
q Extents are allocated for file allocation
q A file consists of one or more extents
Operating System Concepts – 8th Edition 11.20 Silberschatz, Galvin and Gagne ©2009
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