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Chapter 11: Implementing File Systems In-Memory File-System Structures

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Chapter 11: Implementing File Systems In-Memory File-System Structures 11.1 File-System Structure File structure z Logical storage unit z Collection of related information File system resides on secondary storage (disks) File system organized into layers Chapter 11: File control block – storage structure consisting Chapter 11: of information about a file Implementing File Systems Figure 11.2 A typical file-control block Figure 11.1 Layered File System 11.3 Chapter 11: Implementing File Systems In-Memory File-System Structures 11.1 File-System Structure 11.2 File-System Implementation 11.3 Directory Implementation 11.4 Allocation Methods 11.5 Free-Space Management 11.6 Efficiency and Performance 11.7 Recovery 11.8 Log-Structured File Systems 11.9 NFS Figure 11.3 (a) File open (b) File read 11.2 11.4 Virtual File Systems 11.4 Allocation Methods Virtual File Systems (VFS) provide an object-oriented way of implementing file systems. VFS allows the same system call interface (API) to be used for different types of An allocation method refers to how disk blocks are file systems. allocated for files: The API is to the VFS interface, rather than any specific type of file system. 11.4.1 Contiguous allocation 11.4.2 Linked allocation 11.4.3 Indexed allocation Figure 11.4 Schematic View of Virtual File System 11.5 11.7 11.3 Directory Implementation 11.4.1 Contiguous Allocation Linear list of file names with pointer to the data blocks. Each file occupies a set of contiguous blocks on the disk z advantage: simple to program Advantages z disadvantage: time-consuming to execute z Simple – only require starting location (block #) and Hash Table – linear list with hash data structure. length (number of blocks) z Immediate access block i z advantage: decreases directory search time Difficulties z disadvantage: z Finding space for a new file collisions – situations where two file names hash to the same location first fit, best fit external fragmentation fixed size z File size is unknown while created Disadvantages z Wasteful of space (dynamic storage- allocation problem) z Files cannot grow Figure 11.5 contiguous allocation of disk space 11.6 11.8 Extent-Based Systems File-Allocation Table (FAT) Many newer file systems (I.e. Veritas File System) use a modified contiguous allocation scheme Extent-based file systems allocate disk blocks in extents z An extent is a contiguous block of disks z A file consists of one or more extents. Design z initially, a contiguous chunk of space is allocated z if not enough, another chunk (called extent) is added the location of a file’s blocks is recorded as a location and a block, plus a link to the first block of the next extent Issues z Internal fragmentation becomes a problem if the extents are too large z External fragmentation becomes a problem as extents of varying sizes are allocated and deallocated Figure 11.7 File-allocation table 11.9 11.11 11.4.2 Linked Allocation 11.4.3 Indexed Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on disk Brings all pointers per file together into the index block. Logical view. block = pointer Example: 512 bytes / block = 508 bytes of data + 4 bytes of pointer Simple – need only starting address index table Free-space management system – no waste of space Advantages Disadvantages z solve external fragmentation z space require for the pointers z no size-declaration problem unit of cluster (multiple blocks) z support direct access – one pointer for multiple blocks Disadvantages – fast disk access time z Internal fragmentation – internal fragmentation z greater pointer overhead than z slow access to block i linked allocation Figure 11.6 Linked allocation of disk space z low reliability due to crash z Need index table: how large? Figure 11.8 Indexed allocation of disk space Examples: File-allocation table (FAT) – disk- space allocation used by MS-DOS and OS/2 11.10 11.12 Index Block Allocation 11.5 Free-Space Management Index block is normally one disk block Bit vector (n blocks) Mapping from logical to physical in a file of unbounded length (block of 512 words) 01 2 n-1 Linked scheme – Link together several index blocks (no limit on size) … z small header giving file name and a set o the first disk-block addresses Block number calculation z The next address (the last word in the index block) is nil (for a small file) (number of bits per word) * 0 ⇒ block[i] free a pointer to another index block (for a large file) (number of 0-value words) + bit[i] = 1 ⇒ block[i] occupied Multilevel index offset of first 1 bit 678 z a first-level index block to point to a set of second-level index blocks Bit map requires extra space z a second-level index block point to the file blocks z Example: block size = 212 bytes disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes) z Easy to get contiguous files M Linked list (free list): link together all the free disk blocks Two-level Index Example z Cannot get contiguous space easily z No waste of space Grouping outer-index Counting index table file 11.13 11.15 Index Block Allocation: Combined Scheme UNIX (4K bytes per block) Free-Space Management (Cont.) Need to protect: z Pointer to free list keep first 15 pointers of the z Bit map index block in the file’s inode Must be kept on disk z 12 pointers: direct blocks Copy in memory and disk may differ z indirect block Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk z Solution: Set bit[i] = 1 in disk Allocate block[i] Set bit[i] = 1 in memory 11.14 11.16 Linked Free Space List on Disk I/O Without a Unified Buffer Cache 11.17 11.19 11.6 Efficiency and Performance Unified Buffer Cache Efficiency dependent on: A unified buffer cache uses the same page cache to cache both z disk allocation and directory algorithms memory-mapped pages and ordinary file system I/O z types of data kept in file’s directory entry Performance z disk cache – separate section of main memory for frequently used blocks z free-behind and read-ahead – techniques to optimize sequential access z improve PC performance by dedicating section of memory as virtual disk, or RAM disk Page cache z A page cache caches pages rather than disk blocks using virtual memory techniques z Memory-mapped I/O uses a page cache z Routine I/O through the file system uses the buffer (disk) cache z This leads to the following figure 11.18 11.20 11.7 Recovery 11.9 The Sun Network File System (NFS) Consistency checking – compares data in directory structure with An implementation and a specification of a software system for data blocks on disk, and tries to fix inconsistencies accessing remote files across LANs (or WANs) Use system programs to back up data from disk to another storage The implementation is part of the Solaris and SunOS operating device (floppy disk, magnetic tape, other magnetic disk, optical) systems running on Sun workstations using an unreliable datagram protocol (UDP/IP protocol and Ethernet Recover lost file or disk by restoring data from backup 11.21 11.23 11.8 Log Structured File Systems NFS (Cont.) Interconnected workstations viewed as a set of independent machines with Log structured (or journaling) file systems record each update to independent file systems, which allows sharing among these file systems in a the file system as a transaction transparent manner z A remote directory is mounted over a local file system directory The mounted directory looks like an integral subtree of the local file All transactions are written to a log system, replacing the subtree descending from the local directory z A transaction is considered committed once it is written to the z Specification of the remote directory for the mount operation is log 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 z However, the file system may not yet be updated z 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 The transactions in the log are asynchronously written to the file NFS is designed to operate in a heterogeneous environment of different system machines, operating systems, and network architectures; the NFS specifications independent of these media z When the file system is modified, the transaction is removed This independence is achieved through the use of RPC primitives built on top of from the log an External Data Representation (XDR) protocol used between two implementation-independent interfaces If the file system crashes, all remaining transactions in the log must The NFS specification distinguishes between the services provided by a mount still be performed mechanism and the actual remote-file-access services 11.22 11.24 Three Independent File Systems NFS Protocol Provides a set of remote procedure calls for remote file operations. The procedures support the following operations: z searching for a file within a directory z reading a set of directory entries z manipulating links and directories z accessing file attributes z 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 (a) mounts (b) Cascading mounts 11.25 11.27 Three Major Layers
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