Computer Operating System - Lecture 12: Files system implementation - Nguyen Thanh Son

Nội dung text: Computer Operating System - Lecture 12: Files system implementation - Nguyen Thanh Son

1. Chapter’s Content  File System Structure  File System Implementation  Directory Implementation  Allocation Methods  Free-Space Management  Efficiency and Performance  Recovery  Log-Structured File Systems  NFS BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 2
2. Layered File System BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 4
3. 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 6
4. 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 BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 8
5. 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 10
6. 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 12
7. Linked Allocation (Cont.)  Simple – need only starting address  Free-space management system – no waste of space  No random access Q  Mapping LA/511 R 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 14
8. File-Allocation Table BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 16
9. Example of Indexed Allocation BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 18
10. 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 Q table (no limit on size). 1 LA / (512 x 511) R1 Q1 = block of index table R1 is used as follows: Q Q = displacement into block of index table 2 2 R1 / 512 R displacement into block of file: 2 R2 BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 20
11. Indexed Allocation – Mapping (Cont.)  outer-index index table file BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 22
12. Free-Space Management  Bit vector (n blocks) 0 1 2 n-1 0 block[i] free bit[i] = 1 block[i] occupied  Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 24
13. 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 BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 26
14. 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 28
15. 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 30
16. Unified Buffer Cache  A unified buffer cache uses the same page cache to cache both memory- mapped pages and ordinary file system I/O. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 32
17. 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 (floppy disk, magnetic tape).  Recover lost file or disk by restoring data from backup. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 34
18. The Sun Network File System (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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 36
19. 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 BK TP.HCM the actual remote-file-access services. 07-Feb-17 Faculty of Computer Science & Engineering 38
20. Mounting in NFS BK Mounts Cascading mounts TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 40
21. NFS 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 BK TP.HCM mechanisms. 07-Feb-17 Faculty of Computer Science & Engineering 42
22. Schematic View of NFS Architecture BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 44
23. 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 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. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 46