Operating Systems 2010/2011
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1 Operating Systems 2010/2011 File Systems part 2 (ch11, ch17) Shudong Chen 1
2 Recap Tasks, requirements for filesystems Two views: User view File type / attribute / access modes Directory structure OS designers view The API File operations Directory operations Namespaces Namespace construction methods / purpose / joining File sharing File sharing / failure modes / consistency File protection Access list & group TU/e Computer Science, System Architecture and Networking 07/12/2010 2
3 Objectives OS view implementation from designers perspective To describe the details of implementing local file systems and directory structures To discuss block allocation and free-block algorithms and tradeoffs To describe the implementation of remote file systems Distributed file systems TU/e Computer Science, System Architecture and Networking 07/12/2010 3
4 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 4
5 Design tasks To provide efficient and convenient access to secondary-storage devices by allowing data to be stored, located, and retrieved easily. Two design problems defining how the file system should look to the user defining a file and its attributes defining the operations allowed on a file defining the directory structure for organizing files creating algorithms and data structures to map the logical file system onto physical secondary-storage devices 5
6 Layered file system FS is organized into layers Higher layers use the features of lower layers to create new features. Duplication of code is minimized. Layering introduces more overhead which may result in deceased performance. How many layers to use and what each layer should do is a major design challenge. Manage metadata information directory structure (file-organization module) file structure (FCB) Responsible for protection and security Issue generic commands to the appropriate device driver to read and write physical blocks on the disk Translate logical block addresses to physical block addresses Manage free-space Manage memory buffers and caches that hold various file systems, directory and data blocks Transfer information between the main memory and the disk system 6 Consist of device drivers and interrupt handlers
7 File descriptor Metadata includes all of the file-system structure except the actual data (contents of the files) FCB (file control block) storage structure consisting of information about a file ownership, permissions, and locations of the file contents all information about a file in one place eases access, update avoids consistency problems with a unique identifier number to associate with a directory entry Examples Unix: inode table data structure per file NTFS: master file table relational database structure: a row per file TU/e Computer Science, System Architecture and Networking 07/12/2010 7
8 Example: inode (Unix) (a rather dated version) TU/e Computer Science, System Architecture and Networking 07/12/2010 8
9 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 9
10 Layered file system FS is organized into layers Higher layers use the features of lower layers to create new features. Transfer information between the main memory and the disk system Consist of device drivers and interrupt handlers 10
11 On-disk file system structures File system resides on secondary storage (disks) Most disks are divided into partitions Each partition has its own file systems Sector 0 of a disk is called MBR (Master Boot Record) MBR is used to boot the computer The end of MBR contains the partition table Giving starting and ending addresses of each partition One of the partitions is marked as active in the table At boot time BIOS reads in and run the MBR MBR then locates the active partition and reads in the first block (the boot contol block), and execute it Boot control block in turns loads the OS in the partition File system contains information about how to boot an operating system stored there ---- (boot control block) the total number of blocks ---- (volume control block) the number and location of free blocks ---- (volume control block) the directory structure and individual files 11
12 On-disk file system structure (Cont.) Each disk partition contains boot control block typically the first block of a volume contains information needed by the system to boot an OS from that volume can be empty, if the disk does not contain an OS called as boot block in UFS, partition boot sector in NTFS volume control block contains volume details, such as the number of blocks in the partition, the size of the blocks, a free-block count and free-block pointers, and a free-fcb count and FCB pointers. called as superblock in UFS, master file table in NTFS the directory structure and individual files 12
13 Layered file system FS is organized into layers Higher layers use the features of lower layers to create new features. Issue generic commands to the appropriate device driver to read and write physical blocks on the disk Manage memory buffers and caches that hold various file systems, directory and data blocks 13
14 Implementing basic file system Implementation Setup and maintain tables for files that are currently being accessed ( open ) caching of attributes maintaining the temporary attributes access control Exchange reference to descriptor with I/O control TU/e Computer Science, System Architecture and Networking 07/12/
15 In-memory file system structures In-memory directory structure cache holds the directory information of recently accessed directories to speed directory operations System-wide open-file table (OFT) contains a copy of the FCB of each open file, as well as other information, e.g., the number of processes that have the file open. Per-process open-file table contains a pointer to the appropriate entry in the system-wide open-file table, as well as other information, e.g., a pointer to the current location in the file the access mode in which the file is open 15
16 Example: opening a file from an application (OFT) Open File Table Application Kernel open( /home/johan/aap, O_RDWR, O_CREAT) open subdir home of / check permission create data structure descriptors open subdir johan from /home open file Aap of /home/johan / owner: root access: drwxr-xr-x buffers, block, containing entries: home, usr,... /home owner: root access: drwxr-xr-x entries: johan, piet,... /home/johan owner: johan access: drwxr-xr-x entries: OS, Aap,... descriptor (pointer) to data structure /home/johan/aap owner: johan access: -rw size: 1021 blocks, buffers,... 16
17 Open File Table Upon open API call: verify descriptor info against user (process) rights allocate resources: OFT entry, buffers initialize access: read/write pointer copy and maintain information from the i-node return index (a pointer to the appropriate entry) in OFT for subsequent access called as file descriptor in Unix, file handler in Windows in Linux: this index is stored with the process Close flush buffers to file contents and OFT entry to inode release resources (buffer, OFT entry) per-process OFT entry is removed, system-wide OFT entry is decremented Notice: at this level file-sharing may occur two process accessing the same file both with an entry in the OFT results unpredictable can use a file as semaphore ---- the first that creates it has it TU/e Computer Science, System Architecture and Networking 07/12/
18 In-memory file system structures (a) refers to opening a file. (b) refers to reading a file. 18
19 Virtual File Systems Virtual File Systems (VFS) provide an object-oriented way of implementing multiple types of file systems allow 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. VFSs have three major layers: File-system interface layer based on the open(), read(), write(), and close() calls and on file descriptors. VFS interface layer, which servers two important functions: Separates file-system-generic operations from their implementation by defining a clean VFS interface Provides a mechanism for uniquely representing a file throughout a network. Different types of FS and remote file system protocol implementing layer Handle local requests according to their FS types Calls the NFS protocol procedures for remote requests. VFS distinguishes local files from remote ones, and local files are further distinguished according to their file-system types. 19
20 Linux VFS architecture four main object types inode object --- represents an individual file file object --- represents an open file superblock object --- represents an entire file system dentry object --- represents an individual directory entry a set of operations for each object type, e.g., APIs for the file object int open ( ), ssize_t read( ), ssize_t write( ), int mmap( ) a function table lists the addresses of the actual functions that implement these operations every object contains a pointer to the function table complete definition: /usr/include/linux/fs.h TU/e Computer Science, System Architecture and Networking 07/12/
21 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 21
22 Layered file system FS is organized into layers Higher layers use the features of lower layers to create new features. Translate logical block addresses to physical block addresses Manage free-space 22
23 Directory implementation File directory: collection of (entries for) files mapping to access point for the file object (or to an internal address) just a file with internal structure Implementation task Resolving remapping info, e.g. mounting... in fact, the implementation of the name-space Two issues what to store per entry? name + description scale of possibilities ranging from all info (attributes) here to just a pointer to information tradeoff: efficiency-storage policy: never copy volatile information, hence a reference here is better how to organize the entries? efficient insert & delete TU/e Computer Science, System Architecture and Networking 07/12/
24 Data structures for implementation Linear list of file names with pointer to the data blocks simple to program time-consuming to execute --- requires a linear search insertion: first search the directory to be sure that no existing file with same name, then add the new entry to the end. deletion: first search, then release the space allocated to it. Hash Table --- linear list with hash data structure decreases directory search time the hash table takes a value computed from the file name and returns a pointer to the file name in the linear list collisions: situations where two file names hash to the same location the major difficulties fixed size the dependence of the hush function on that size 24
25 Data structures for implementation Variable sized linked list length/value encoding B-trees insert and delete logarithmic Search by yourself for details good for searching, bad for sequential access tunable to e.g. disk-block size B+ trees: nothing stored in internal nodes good in both somewhat more overhead as internal nodes are empty TU/e Computer Science, System Architecture and Networking 07/12/
26 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 26
27 Allocation methods An allocation method refers to how disk blocks are allocated for files. Design tasks Disk space is utilized effectively Files can be accessed quickly Three common approaches are possible Contiguous allocation Linked list allocation Indexed allocation 27
28 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 Pros fast sequential access: less seeks between consecutive disk blocks easy random access: can easily calculate the disk location of any file block Cons external fragmentation wastes space if user declares a bigger size hard to grow files 28
29 Extent-Based Systems Many newer file systems (i.e. Veritas File System) use a modified contiguous allocation scheme a contiguous chunk of space is allocated initially if that amount proves not to be large enough, another chunk of contiguous space, known as an extent, is added the location of a file s blocks is recorded as location and a block count plus a link to the first block of the next extent a file consists of one or more extents 29
30 Linked List Allocation Keeps each file as a linked list of disk blocks The directory contains a pointer to the first and last blocks of the file First word of each block points to the next one Disk blocks need not be adjacent May be scattered anywhere on the disk 30
31 Linked List Allocation Pros no space lost to disk fragmentation solves the external fragmentation and size-declaration problems of contiguous allocation fast sequential access Cons extremely slow random access amount of data in block is not a power of 2 How to solve the problems? use File Allocation Table (FAT) take the pointer word from each disk block putting it in a table in memory 31
32 File-Allocation Table Linked allocation cannot support efficient direct access! Pointers to the blocks are scattered with the blocks themselves all over the disk. Pointers must be retrieved in order. Linked list allocation using a file allocation table in RAM 32
33 Indexed Allocation Brings all pointers together into the index block. User (or system) declares max number of blocks in file System allocates a file header with an array of pointers big enough to point to that number of blocks the header is also called an inode The file header is load into memory when open Equivalent to a page table for MMU 33
34 Indexed Allocation Pros can easily grow file up to the number of blocks allocated in file header easy random access Cons lots of seeks for sequential access hard to grow file bigger than initially allocated in the file header 34
35 Multi-level Indexed Allocation How to grow files sizes? The solution is to use multi-level index files direct blocks contain addresses of blocks that contain data of the file single indirect block is an index block containing the addresses of blocks that contain pointers to the actual data blocks double indirect block triple indirect block Combined scheme of Unix 35
36 Multi-level Indexed Allocation Pros and cons files can easily expand small files don t pay full overhead of deep trees Cons lots of indirect blocks for big files lots of seeks for sequential access 36
37 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 37
38 Bit vector (bit map) Each block is represented by 1 bit n-1 bit[ i ] = 0 block[ i ] allocated 1 block[ i ] free Relatively simple Efficient in finding the first free block or n consecutive free blocks The first free block number calculation: (number of bits per word) X (number of 0-value words) + offset of first 1 bit 38
39 Linked Free Space List on Disk Standard linked-list approach link together all the free disk blocks keeping a pointer to the first free block in a special location in the disk and caching it in memory the first block contains a pointer to the next free disk, and so on Grouping stores the address of n free blocks in the first free block the first n-1 block are free the last block contains the addresses of another n free blocks Counting keep the address of the first free block and the number (n) of free contiguous blocks that follow the first block 39
40 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 40
41 Efficiency Efficiency in disk use dependent on: disk allocation and directory algorithms in use, e.g., inodes space lost --- preallocated to a volume however, improved file system performance --- reduced seek time by keeping a file s data blocks near the inode block types of data kept in file s directory entry, e.g., last access date to determine whether the file needs to be backed up whenever a file is read, a field in the directory structure must be written to pointer size static or dynamic table entry de-allocation (OFT) 41
42 Performance Ways to improve PC performance disk cache separate section of main memory for frequently used blocks free-behind and read-ahead techniques to optimize sequential access free behind remove a page from the buffer as soon as the next page is requested assume that the previous pages are not likely to be used again and waste buffer space read-ahead a requested page and several subsequent pages are read and cached assume that these pages are likely to be requested after the current page is processed dedicating section of memory as virtual disk, or RAM disk 42
43 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 43
44 Remaining issues Reliability errors loss of a block create a file of lost blocks recover lost file or disk by restoring data from backup inconsistencies consistency checking compares data in directory structure with data blocks on disk, and tries to fix inconsistencies replicate data repeat the basic tables store several versions avoid errors journaling: record a log of updates write (to a file / region of the disk) the operation to be performed before actually doing it journal: small, circular buffer TU/e Computer Science, System Architecture and Networking 07/12/
45 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 45
46 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems Please read by yourselves TU/e Computer Science, System Architecture and Networking 07/12/
47 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 (V3) 47
48 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 manner A 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 directory Specification 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 manner Subject to access-rights accreditation, potentially any file system (or directory within a file system), can be mounted remotely on top of any local directory 48
49 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 the actual remote-file-access services 49
50 Three Independent File Systems 50
51 Mounting in NFS Mounts Cascading mounts 51
52 NFS Mount Protocol Establishes initial logical connection between server and client Mount operation includes name of remote directory to be mounted and name of server machine storing it Mount 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 handle a key for further accesses File handle a file-system identifier, and an inode number to identify the mounted directory within the exported file system The mount operation changes only the user s view and does not affect the server side 52
53 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 (NFS V4 is just coming available very different, stateful) 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 mechanisms 53
54 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 types The VFS activates file-system-specific operations to handle local requests according to their file-system types Calls the NFS protocol procedures for remote requests NFS service layer bottom layer of the architecture Implements the NFS protocol 54
55 Schematic View of NFS Architecture 55
56 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 vnode To make lookup faster, a directory name lookup cache on the client s side holds the vnodes for remote directory names 56
57 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 57
58 Agenda File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems TU/e Computer Science, System Architecture and Networking 07/12/
59 Background Distributed file system (DFS) a distributed implementation of the classical time-sharing model of a file system, where multiple users share files and storage resources A DFS manages set of dispersed storage devices Overall storage space managed by a DFS is composed of different, remotely located, smaller storage spaces There is usually a correspondence between constituent storage spaces and sets of files 59
60 DFS Structure Service software entity running on one or more machines and providing a particular type of function to a priori unknown clients Server service software running on a single machine Client process that can invoke a service using a set of operations that forms its client interface A client interface for a file service is formed by a set of primitive file operations (create, delete, read, write) Client interface of a DFS should be transparent, i.e., not distinguish between local and remote files 60
61 Naming Naming mapping between logical and physical objects Comparing to a conventional file system, in a DFS the naming mapping is expanded to included the specific machine on whose disk the file is stored For a file being replicated in several sites, the mapping returns a set of the locations of this file s replicas Symbolic link into other namespace 61
62 Mounting Example: netmount /dev/hda1 server1:/usr/local Current namespace filesystem on server1 bin lib TU/e Computer Science, System Architecture and Networking Store closure (reference to /dev/hda1, filesystem type,...) with prefix file access means network traffic 07/12/
63 Naming Schemes Three Main Approaches Files named by combination of their host name and local name; guarantees a unique system-wide name Attach remote directories to local directories, giving the appearance of a coherent directory tree; only previously mounted remote directories can be accessed transparently Total integration of the component file systems A single global name structure spans all the files in the system If a server is unavailable, some arbitrary set of directories on different machines also becomes unavailable 63
64 DFS organization Both client and server have a file system Server file system task: serve requests from the network Client file system task: serve requests from the application transparently include server file system TU/e Computer Science, System Architecture and Networking 07/12/
65 Choices Design choices concern the layer where the connection client-server is put 1. Application: client just accesses the remote file system as just another application needs a server application on the other side comparable to web-servers 2. Logical file system, file-organization module and basic file system client uses the remote file system as a process, via descriptor (references) 3. I/O control client uses the remote file system as a block storage mechanism 65
66 Stateful & stateless File Service Two approaches to store server-side information when a client accesses remote files: Stateful: the server tracks each file being accessed by each client Stateless: the server only provides required blocks without knowledge of how they are used 66
67 Stateful File Service Mechanism Client opens a file Server fetches information about the file from its disk, stores it in its memory, and gives the client a connection identifier unique to the client and the open file Identifier is used for subsequent accesses until the session ends Server must reclaim the main-memory space used by clients who are no longer active Increased performance Fewer disk accesses Stateful server knows if a file was opened for sequential access and can thus read ahead the next blocks 67
68 Stateful server Server implements basic file system 68
69 Stateless File Server Avoids state information by making each request selfcontained Each request identifies the file and position in the file No need to establish and terminate a connection by open and close operations 69
70 Stateless server Server implements low-level data access 70
71 Distinctions between stateful & stateless Service Failure Recovery A stateful server loses all its volatile state in a crash Restore state by recovery protocol based on a dialog with clients, or abort operations that were underway when the crash occurred Server needs to be aware of client failures in order to reclaim space allocated to record the state of crashed client processes (orphan detection and elimination) With stateless server, the effects of server failure sand recovery are almost unnoticeable A newly reincarnated server can respond to a selfcontained request without any difficulty 71
72 Distinctions (Cont.) Penalties for using the robust stateless service: longer request messages slower request processing additional constraints imposed on DFS design Some environments require stateful service A server employing server-initiated cache validation cannot provide stateless service, since it maintains a record of which files are cached by which clients UNIX use of file descriptors and implicit offsets is inherently stateful; servers must maintain tables to map the file descriptors to inodes, and store the current offset within a file 72
73 Caching & Consistency Retaining recently accessed disk blocks in a cache to reduce network traffic remote accesses to the same information can be handled locally Is locally cached copy of the data consistent with the master copy? Client-initiated approach Client initiates a validity check Server checks whether the local data are consistent with the master copy Server-initiated approach Server records, for each client, the (parts of) files it caches When server detects a potential inconsistency, it must react 73
74 Summary File system structure File system implementation Directory implementation Allocation methods Free space management Efficiency and performance Recovery NFS Distributed file systems 74
75 Exercises Ch11-3, 5, 6, 10, 12 Ch17-1, 2, 9 Check the website TU/e Computer Science, System Architecture and Networking 07/12/
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