File Management Lecture 3 What is a file
- Slides: 44
File Management Lecture 3
What is a file? • File is term applied to anything held on secondary storage • Includes programs (source and executable) text files such as word documents, spreadsheet files, database files etc.
File naming • MS DOS format is generally up to eight character name followed by a dot and then a three character extension. • Example Letter 1. doc , hello. exe , stock. dat • Linux/Unix file naming is more flexible can have something like myfile. example. doc normally. bin files are equivelent to exe files in windows.
Objectives of O. S. • Hide complexity of how files are saved from user • Problems with physical addressing, perform error checking, i/o tasks to storage. • Develop file management strategies • Explore files and folders • Create, name, copy, move, and delete folders • Name, copy, move, and delete files • Work with compressed files
Organizing Files and Folders • A file, or document, is a collection of data that has a name and is stored in a computer • You organize files by storing them in folders • Disks contain folders that hold documents, or files – Floppy disks – Zip disks – Compact Discs (CDs) – Hard Disks • Removable disks are inserted into a drive
Organizing Files and Folders
Understanding the Need for Organizing Files and Folders • Windows organizes the folders and files in a hierarchy, or file system • Windows stores folders and important files that it needs when you turn on the computer in the root directory • Folders stored within other folders are called subfolders
Understanding the Need for Organizing Files and Folders
Directories • A directory is a logical grouping of files • All modern operating systems have a directory structure • Why security and housekeeping on system • Example on Linux system only root user will have access to sbin directory.
Linux/Unix file structure
File Management System • Provides a logical view for the user and hides the physical implementation • Where a file is located and how it is stored on disk is role of OS • Manages directory structures and space allocation for each I/O device • Permits manipulation of data within a file • Requests data transfers from I/O device drivers • File security and protection of file integrity
File Management and I/O Functions Separation between the two allows 1. I/O devices can change while keeping the file system the same 2. Redirecting of data is simple
File Manager Request Handling
File Storage • Over time file sizes change this can be a problem for OS. As files reduce get deleted, compressed etc spaces develop on disk – fragmentation. • OS provides fixed size blocks for storing data, called cluster. • Problem is clusters are often not sequential.
File Access Methods • Sequential Access – File is read in sequence from beginning to end – Majority of all files – Program source and binary files • Random Access – Assumes file is made up of fixed length logical records – Hashing is a common method used to calculate the location of an internal logical record • Indexed Access – Additional means for accessing and viewing records in a file – Key indexes
Physical File Storage • Contiguous • Non-contiguous – Linked – Indexed • Examples – DOS/Windows FAT – UNIX i-nodes – Windows NTFS – Free space management
Contiguous Storage Allocation • Assign blocks (all in a row) to hold the file • Access is simple for both sequential and random methods • Disadvantages – – Space must be large enough Have to take into account file growth May need to be moved if it outgrows its space Fragmentation of disk • Allocation strategies to minimize fragmentation – First-fit, best-fit • Eventually disk becomes fragmented
Contiguous Storage Allocation
Linked Allocation • • Non-contiguous Each block contains a link to the next physical block Variant – links in both directions Advantages – no fragmentation – Adding to a file is easy • Disadvantages – – Not usable for random access Additional disk head searching Overhead in storing the pointers Recovery of a defective block is difficult
Linked Allocation
MS-DOS FAT • File Allocation Table (FAT) • Table contains the first block of each file on the disk or disk partition • Successive blocks contain a link to the next block • Requires a tremendous amount of space • File integrity can be easily compromised
MS-DOS FAT Linked Allocation and File Allocation Table
Indexed Allocation Index blocks for indexed allocation of linked files shown in MS-DOS FAT example
Indexed Allocation • Non-contiguous • All link pointers are stored together in a single block called the index block • One index block per file • Advantages – No fragmentation – Can be used for random access • Disadvantage – Slower due to additional access of the index block – Additional disk head searching – Recovery of a defective block is difficult
Unix i-nodes • Indexed file allocation • Index block contains – – – File attributes 10 direct blocks 1 single indirect 1 double indirect 1 triple indirect • Advantages – Fast for small blocks – Can accommodate very large files – 100’s of gigabytes
Unix i-nodes
Windows 2000 - NTFS • Dynamically sized volumes • Volumes may be a fraction of a disk or span many disks • Master File Table (MFT) of 1 kb records – 1 st 16 records are attributes of the MFT ie system files used to manage the volume – Each file has an MFT entry
NTFS Volume Layout
Other Secondary Storage Allocation • Tape Allocation – Not practical to reallocate space in the middle of the tape – Files that grow must be re-written – Files are stored contiguously whenever possible • CD-ROM and DVD-ROM Allocation – Block system described in Chapter 10 – Eight levels of subdirectories – Directory format similar to MS-DOS although extensions permit longer filenames and deeper subdirectory levels – Files can be stored non-contiguously
Directory Structure • Provides a means of organization so that files can be located easily and efficiently • Hide the physical devices from the logical view of the files • Partitions – Independent subsections of a device • Volume – Directory structure for a particular partition – Needs to be mounted to be incorporated into the overall file system structure • Contain file attributes
Tree-Structure Directory • Hierarchical with a top-level root directory from which all other directories stem • All directories and files have names • Separator – Used to indicate subdirectories and files located in a directory – / UNIX – DOS, Windows • Pathname – Absolute – full pathname starting from the root directory – Relative – pathname is created starting from the current directory • Search Paths – Directory locations that the operating system uses to locate files
Tree-Structure Directory
Linux/Unix Tree Structure
RAID • • • Redundant Array of Independent Disks Redundant Array of Inexpensive Disks 6 levels in common use Not a hierarchy Set of physical disks viewed as single logical drive by O/S • Data distributed across physical drives • Can use redundant capacity to store parity information
RAID 0 • • No redundancy Data striped across all disks Round Robin striping Increase speed – Multiple data requests probably not on same disk – Disks seek in parallel – A set of data is likely to be striped across multiple disks
RAID 1 • • • Mirrored Disks Data is striped across disks 2 copies of each stripe on separate disks Read from either Write to both Recovery is simple – Swap faulty disk & re-mirror – No down time • Expensive
RAID 2 • Disks are synchronized • Very small stripes – Often single byte/word • Error correction calculated across corresponding bits on disks • Multiple parity disks store Hamming code error correction in corresponding positions • Lots of redundancy – Expensive – Not used
RAID 3 • Similar to RAID 2 • Only one redundant disk, no matter how large the array • Simple parity bit for each set of corresponding bits • Data on failed drive can be reconstructed from surviving data and parity info • Very high transfer rates
RAID 4 • • Each disk operates independently Good for high I/O request rate Large stripes Bit by bit parity calculated across stripes on each disk • Parity stored on parity disk
RAID 5 • • • Like RAID 4 Parity striped across all disks Round robin allocation for parity stripe Avoids RAID 4 bottleneck at parity disk Commonly used in network servers • N. B. DOES NOT MEAN 5 DISKS!!!!!
RAID 6 • Two parity calculations • Stored in separate blocks on different disks • User requirement of N disks needs N+2 • High data availability – Three disks need to fail for data loss – Significant write penalty
RAID 0, 1, 2
RAID 3 & 4
RAID 5 & 6
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