Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 11: File System Implementation

11.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter 11: File System Implementation File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery NFS Example: WAFL File System

11.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Objectives To describe the details of implementing local file systems and directory structures To describe the implementation of remote file systems To discuss block allocation and free-block algorithms and trade-offs

11.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File-System Structure File structure Logical storage unit Collection of related information File system organized into layers File system resides on secondary storage (disks) Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily File control block – storage structure consisting of information about a file Device driver controls the physical device

11.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Layered File System

11.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File-System Implementation Boot control block contains info needed by system to boot OS from that volume Volume control block contains volume details Directory structure organizes the files Per-file File Control Block (FCB) contains many details about the file

11.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Typical File Control Block

11.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition In-Memory File System Structures The following figure illustrates the necessary file system structures provided by the operating systems. Figure 12-3(a) refers to opening a file. Figure 12-3(b) refers to reading a file.

11.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition In-Memory File System Structures

11.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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.

11.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Schematic View of Virtual File System

11.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Directory Implementation 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

11.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition How to organize files on disk (Allocation) An allocation method refers to how disk blocks are allocated for files: Goals: Maximize sequential performance Easy random access to file Easy management of file (growth, truncation, etc) Contiguous allocation Linked allocation Indexed allocation

11.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 Random access Wasteful of space (dynamic storage-allocation problem) Files cannot grow

11.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation Use continuous range of blocks in logical block space Analogous to base+bounds in virtual memory User says in advance how big file will be (disadvantage) Search bit-map for space using best fit/first fit What if not enough contiguous space for new file? File Header Contains: First block/LBA in file File size (# of blocks) Pros: Fast Sequential Access, Easy Random access Cons: External Fragmentation/Hard to grow files Free holes get smaller and smaller Could compact space, but that would be really expensive

11.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation Mapping from logical to physical LA/512 Q R Block to be accessed = ! + starting address Displacement into block = R

11.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation of Disk Space

11.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Extent-Based Systems Many 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

11.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. pointer block =

11.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked List Approach Each block, pointer to next on disk Pros: Can grow files dynamically, Free list same as file Cons:Bad Sequential Access (seek between each block), Unreliable (lose block, lose rest of file) Serious Con: Bad random access!!!! Technique originally from Alto (First PC, built at Xerox) No attempt to allocate contiguous blocks MSDOS used a similar linked approach Links not in pages, but in the File Allocation Table (FAT)  FAT contains an entry for each block on the disk  FAT Entries corresponding to blocks of file linked together Compare with Linked List Approach:  Sequential access costs more unless FAT cached in memory  Random access is better if FAT cached in memory Null File Header

11.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 Qth 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. LA/511 Q R

11.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation

11.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File-Allocation Table

11.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation Brings all pointers together into the index block Logical view index table

11.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Example of Indexed Allocation

11.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation (Cont.) Need index table Random access Dynamic access without external fragmentation, but have overhead of index block Mapping from logical to physical in a file of maximum size of 256K words and block size of 512 words. We need only 1 block for index table LA/512 Q R Q = displacement into index table R = displacement into block

11.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation – Mapping (Cont.) Mapping from logical to physical in a file of unbounded length (block size of 512 words) Linked scheme – Link blocks of index table (no limit on size) LA / (512 x 511) Q1Q1 R1R1 Q 1 = block of index table R 1 is used as follows: R 1 / 512 Q2Q2 R2R2 Q 2 = displacement into block of index table R 2 displacement into block of file:

11.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation – Mapping (Cont.) Two-level index (maximum file size is ) LA / (512 x 512) Q1Q1 R1R1 Q 1 = displacement into outer-index R 1 is used as follows: R 1 / 512 Q2Q2 R2R2 Q 2 = displacement into block of index table R 2 displacement into block of file:

11.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation – Mapping (Cont.)  outer-index index table file

11.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Combined Scheme: UNIX UFS (4K bytes per block)

11.31 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management Bit vector (n blocks) … 012n-1 bit[i] =  0  block[i] free 1  block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

11.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) Bit map requires extra space Example: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes) Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space Grouping Counting

11.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) 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

11.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Directory Implementation 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

11.35 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Free Space List on Disk

11.36 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Efficiency and Performance Efficiency dependent on: disk allocation and directory algorithms types of data kept in file’s directory entry Performance disk cache – separate section of main memory for frequently used blocks free-behind and read-ahead – techniques to optimize sequential access improve PC performance by dedicating section of memory as virtual disk, or RAM disk

11.37 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Page 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) cache This leads to the following figure

11.38 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition I/O Without a Unified Buffer Cache

11.39 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory- mapped pages and ordinary file system I/O

11.40 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition I/O Using a Unified Buffer Cache

11.41 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 (magnetic tape, other magnetic disk, optical) Recover lost file or disk by restoring data from backup

11.42 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Log Structured File Systems Log 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

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 11