operatingsystem《操作系统》ch11-filesystemimplementa.ppt

Chapter 11:File System Implementation,Chapter Objectives,To describe the details of implementing local file systems and directory structuresTo describe the implementation of remote file systemsTo discuss block allocation and free-block algorithms and trade-offs,Content Overview,File-System StructureFile-System Implementation Directory ImplementationAllocation MethodsFree-Space Management Efficiency and PerformanceRecoveryLog-Structured File SystemsNFSExample:WAFL File System,11.1 File-System Structure,File structureLogical storage unitCollection of related informationFile system resides on secondary storage(disks)File system organized into layersFile control block storage structure consisting of information about a file,Layered File System,11.2 A Typical File Control Block,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.,In-Memory File System Structures,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.,Schematic View of Virtual File System,11.3 Directory Implementation,Linear list of file names with pointer to the data blocks.simple to programtime-consuming to executeHash Table linear list with hash data structure.decreases directory search timecollisions situations where two file names hash to the same locationfixed size,11.4 Allocation Methods,An allocation method refers to how disk blocks are allocated for files:Contiguous allocationLinked allocationIndexed allocation,Contiguous Allocation,Each file occupies a set of contiguous blocks on the diskSimple only starting location(block#)and length(number of blocks)are requiredRandom accessWasteful of space(dynamic storage-allocation problem)Files cannot grow,Contiguous Allocation of Disk Space,Extent-Based Systems,Many newer file systems(I.e.Veritas File System)use a modified contiguous allocation schemeExtent-based file systems allocate disk blocks in extentsAn extent is a contiguous block of disksExtents are allocated for file allocationA file consists of one or more extents.,Linked Allocation,Each file is a linked list of disk blocks:blocks may be scattered anywhere on the disk.,Linked Allocation(Cont.),Simple need only starting addressFree-space management system no waste of space No random access,File-allocation table(FAT)disk-space allocation used by MS-DOS and OS/2.,Linked Allocation,File-Allocation Table,Indexed Allocation,Brings all pointers together into the index block.Logical view.,index table,Example of Indexed Allocation,Indexed Allocation(Cont.),Need index tableRandom accessDynamic access without external fragmentation,but have overhead of index block.,Indexed Allocation Mapping(Cont.),outer-index,index table,file,Combined Scheme:UNIX(4K bytes per block),11.5 Free-Space Management,Bit vector(n blocks),0,1,2,n-1,biti=,1 blocki free0 blocki occupied,Block number calculation,(number of bits per word)*(number of 0-value words)+offset of first 1 bit,Free-Space Management(Cont.),Bit map requires extra spaceExample:block size=212 bytesdisk size=230 bytes(1 gigabyte)n=230/212=218 bits(or 32K bytes)Easy to get contiguous files Linked list(free list)Cannot get contiguous space easilyNo waste of spaceGrouping Counting,Linked Free Space List on Disk,11.6 Efficiency and Performance,Efficiency dependent on:disk allocation and directory algorithmstypes of data kept in files directory entryPerformancedisk cache separate section of main memory for frequently used blocksfree-behind and read-ahead techniques to optimize sequential accessimprove PC performance by dedicating section of memory as virtual disk,or RAM disk,Page Cache,A page cache caches pages rather than disk blocks using virtual memory techniquesMemory-mapped I/O uses a page cacheRoutine I/O through the file system uses the buffer(disk)cacheThis leads to the following figure,I/O Without a Unified Buffer Cache,Unified Buffer Cache,A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O,I/O Using a Unified Buffer Cache,11.7 Recovery,Consistency checking compares data in directory structure with data blocks on disk,and tries to fix inconsistenciesUse system programs to 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,11.8 Log Structured File Systems,Log structured(or journaling)file systems record each update to the file system as a transactionAll transactions are written to a log A transaction is considered committed once it is written to the logHowever,the file system may not yet be updatedThe transactions in the log are asynchronously written to the file system When the file system is modified,the transaction is removed from the logIf the file system crashes,all remaining transactions in the log must still be performed,11.9 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,NFS(Cont.),Interconnected workstations viewed as a set of independent machines with independent file systems,which allows sharing among these file systems in a transparent mannerA remote directory is mounted over a local file system directory The mounted directory looks like an integral subtree of the local file system,replacing the subtree descending from the local directorySpecification of the remote directory for the mount operation is nontransparent;the host name of the remote directory has to be provided Files in the remote directory can then be accessed in a transparent mannerSubject to access-rights accreditation,potentially any file system(or directory within a file system),can be mounted remotely on top of any local directory,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 mediaThis independence is achieved through the use of RPC primitives built on top of an External Data Representation(XDR)protocol used between two implementation-independent interfacesThe NFS specification distinguishes between the services provided by a mount mechanism and the actual remote-file-access services,Three Independent File Systems,Mounting in NFS,Mounts,Cascading mounts,NFS Mount Protocol,Establishes initial logical connection between server and clientMount operation includes name of remote directory to be mounted and name of server machine storing itMount request is mapped to corresponding RPC and forwarded to mount server running on server machine Export list specifies local file systems that server exports for mounting,along with names of machines that are permitted to mount them Following a mount request that conforms to its export list,the server returns a file handlea key for further accessesFile handle a file-system identifier,and an inode number to identify the mounted directory within the exported file systemThe mount operation changes only the users view and does not affect the server side,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 attributesreading and writing filesNFS servers are stateless;each request has to provide a full set of arguments(NFS V4 is just coming available very different,stateful)Modified data must be committed to the servers disk before results are returned to the client(lose advantages of caching)The NFS protocol does not provide concurrency-control mechanisms,Three Major Layers of NFS Architecture,UNIX file-system interface(based on the open,read,write,and close calls,and file descriptors)Virtual File System(VFS)layer distinguishes local files from remote ones,and local files are further distinguished according to their file-system typesThe VFS activates file-system-specific operations to handle local requests according to their file-system types Calls the NFS protocol procedures for remote requestsNFS service layer bottom layer of the architectureImplements the NFS protocol,Schematic View of NFS Architecture,NFS Path-Name Translation,Performed by breaking the path into component names and performing a separate NFS lookup call for every pair of component name and directory vnodeTo make lookup faster,a directory name lookup cache on the clients side holds the vnodes for remote directory names,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 attributesCached file blocks are used only if the corresponding cached attributes are up to dateFile-attribute cache the attribute cache is updated whenever new attributes arrive from the serverClients do not free delayed-write blocks until the server confirms that the data have been written to disk,11.10 Example:WAFL File System,Used on Network Appliance“Filers”distributed file system appliances“Write-anywhere file layout”Serves up NFS,CIFS,http,ftpRandom I/O optimized,write optimizedNVRAM for write cachingSimilar to Berkeley Fast File System,with extensive modifications,The WAFL File Layout,Snapshots in WAFL,11.02,End of Chapter 11,