Chapter 12 File System Implementation n File System

Chapter 12: File System Implementation n File System Structure n File System Implementation n Directory Implementation n Allocation Methods n Free-Space Management n Efficiency and Performance n Recovery n Log-Structured File Systems n NFS Operating System Concepts 12. 1 Silberschatz, Galvin and Gagne 2002

File-System Structure n File structure F Logical storage unit F Collection of related information n File system resides on secondary storage (disks). n File system organized into layers. n File control block – storage structure consisting of information about a file. Operating System Concepts 12. 2 Silberschatz, Galvin and Gagne 2002

Layered File System Operating System Concepts 12. 3 Silberschatz, Galvin and Gagne 2002

A Typical File Control Block Operating System Concepts 12. 4 Silberschatz, Galvin and Gagne 2002

In-Memory File System Structures open Read/write Operating System Concepts 12. 5 Silberschatz, Galvin and Gagne 2002

Virtual File Systems n Virtual File Systems (VFS) provide an object-oriented way of implementing file systems. n VFS allows the same system call interface (the API) to be used for different types of file systems. n The API is to the VFS interface, rather than any specific type of file system. Operating System Concepts 12. 6 Silberschatz, Galvin and Gagne 2002

User program & Kernel, VSF interface Note: This picture Operating System Concepts is excerpted from Write a Linux Hardware Device Driver, Andrew. Galvin O’Shauqhnessy, Unix world Silberschatz, and Gagne 2002 12. 7

Schematic View of Virtual File System Operating System Concepts 12. 8 Silberschatz, Galvin and Gagne 2002

Directory Implementation n Linear list of file names with pointer to the data blocks. F simple to program F time-consuming to execute n Hash Table – linear list with hash data structure. F decreases directory search time F collisions – situations where two file names hash to the same location F fixed size F An improved implementation is a hash table with a linked list (entry) 4 Collision free 4 Can grow Operating System Concepts 12. 9 Silberschatz, Galvin and Gagne 2002

Allocation Methods n An allocation method refers to how disk blocks are allocated for files: n Contiguous allocation n Linked allocation n Indexed allocation Operating System Concepts 12. 10 Silberschatz, Galvin and Gagne 2002

Contiguous Allocation n Each file occupies a set of contiguous blocks on the disk. n Simple – only starting location (block #) and length (number of blocks) are required. n Random access. n Wasteful of space (dynamic storage-allocation problem). n Files cannot grow. Operating System Concepts 12. 11 Silberschatz, Galvin and Gagne 2002

Contiguous Allocation of Disk Space Operating System Concepts 12. 12 Silberschatz, Galvin and Gagne 2002

Linked Allocation n Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. block Operating System Concepts = pointer 12. 14 Silberschatz, Galvin and Gagne 2002

Linked Allocation Operating System Concepts 12. 15 Silberschatz, Galvin and Gagne 2002

Linked Allocation (Cont. ) n Simple – need only starting address n Free-space management system – no waste of space n No random access n Space waste for pointer (e. g. 4 byte of 512 B) n Reliability Operating System Concepts 12. 16 Silberschatz, Galvin and Gagne 2002

File-Allocation Table Improved linked method: File-allocation table (FAT) – diskspace allocation used by MS-DOS and OS/2. Operating System Concepts 12. 17 Silberschatz, Galvin and Gagne 2002

Indexed Allocation n Brings all pointers together into the index block. n Logical view. index table Operating System Concepts 12. 18 Silberschatz, Galvin and Gagne 2002

Example of Indexed Allocation Operating System Concepts 12. 19 Silberschatz, Galvin and Gagne 2002

Indexed Allocation (Cont. ) n Need index table n Random access n Dynamic access without external fragmentation, but have overhead of index block. Operating System Concepts 12. 20 Silberschatz, Galvin and Gagne 2002

Indexed Allocation – Mapping (Cont. ) outer-index table Operating System Concepts 12. 21 file Silberschatz, Galvin and Gagne 2002

Combined Scheme: UNIX (4 K bytes per block) Operating System Concepts 12. 22 Silberschatz, Galvin and Gagne 2002

Free-Space Management n Bit vector (n blocks) 0 1 2 n-1 … bit[i] = Operating System Concepts 0 block[i] free 1 block[i] occupied 12. 24 Silberschatz, Galvin and Gagne 2002

Free-Space Management (Cont. ) n Bit map requires extra space. Example: block size = 212 bytes (4 K) disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32 K bytes) F Easy to get contiguous files n Linked list (free list) F Cannot get contiguous space easily F No waste of space n Grouping F Large free blocks can be quickly found n Counting Operating System Concepts 12. 25 Silberschatz, Galvin and Gagne 2002

Free-Space Management (Cont. ) n Need to protect: F Pointer to free list F Bit map 4 Must be kept on disk 4 Copy in memory and disk may differ. Operating System Concepts 12. 26 Silberschatz, Galvin and Gagne 2002

Linked Free Space List on Disk Operating System Concepts 12. 27 Silberschatz, Galvin and Gagne 2002

Efficiency and Performance n Efficiency dependent on: F disk allocation and directory algorithms F types of data kept in file’s directory entry n Performance F disk cache – separate section of main memory for frequently used blocks F free-behind and read-ahead – techniques to optimize sequential access F improve PC performance by dedicating section of memory as virtual disk, or RAM disk. Operating System Concepts 12. 28 Silberschatz, Galvin and Gagne 2002

Recovery n Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies. n Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape). n Recover lost file or disk by restoring data from backup. Operating System Concepts 12. 34 Silberschatz, Galvin and Gagne 2002

Log Structured File Systems n Log structured (or journaling) file systems record each update to the file system as a transaction. n 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. n 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. n If the file system crashes, all remaining transactions in the log must still be performed. Operating System Concepts 12. 35 Silberschatz, Galvin and Gagne 2002

The Sun Network File System (NFS) n An implementation and a specification of a software system for accessing remote files across LANs (or WANs). n The implementation is part of the Solaris and Sun. OS operating systems running on Sun workstations using an unreliable datagram protocol (UDP/IP protocol and Ethernet. n Interconnected workstations viewed as a set of independent machines with independent file systems, which allows sharing among these file systems in a transparent manner. Operating System Concepts 12. 36 Silberschatz, Galvin and Gagne 2002

NFS (Cont. ) n NFS is designed to operate in a heterogeneous environment of different machines, operating systems, and network architectures; the NFS specifications independent of these media. n This independence is achieved through the use of RPC primitives built on top of an External Data Representation (XDR) protocol used between two implementationindependent interfaces. n The NFS specification distinguishes between the services provided by a mount mechanism and the actual remotefile-access services. Operating System Concepts 12. 37 Silberschatz, Galvin and Gagne 2002

Three Major Layers of NFS Architecture n UNIX file-system interface (based on the open, read, write, and close calls, and file descriptors). n Virtual File System (VFS) layer – distinguishes local files from remote ones, and local files are further distinguished according to their file-system types. F The VFS activates file-system-specific operations to handle local requests according to their file-system types. F Calls the NFS protocol procedures for remote requests. n NFS service layer – bottom layer of the architecture; implements the NFS protocol. Operating System Concepts 12. 40 Silberschatz, Galvin and Gagne 2002

Schematic View of NFS Architecture Operating System Concepts 12. 41 Silberschatz, Galvin and Gagne 2002

NFS Protocol n Provides a set of remote procedure calls for remote file operations. The procedures support the following operations: F searching for a file within a directory F reading a set of directory entries F manipulating links and directories F accessing file attributes F reading and writing files n NFS servers are stateless; each request has to provide a full set of arguments. n Modified data must be committed to the server’s disk before results are returned to the client (lose advantages of caching). n The NFS protocol does not provide concurrency-control mechanisms. Operating System Concepts 12. 42 Silberschatz, Galvin and Gagne 2002