Objectives Learn what a file system does Understand the FAT file system and its advantages and disadvantages Understand the NTFS file system and its advantages and disadvantages Compare various file systems Connecting with Computer Science 2 Objectives continued Learn how sequential and random file access work See how hashing is used Understand how hashing algorithms are created Connecting with Computer Science 3 What Does a File System Do Responsible for creating manipulating renaming copying and removing files to and from a storage device Organizes files into common storage units called directories Keeps track of where files and directories are located Assists users by relating files and folders to the physical structure of the storage medium Connecting with Computer Science 4 Figure 10 1 Files and directories in a file system are similar to documents and folders in a filing cabinet Connecting with Computer Science 5 Storage Mediums A hard disk or drive is the most common storage medium for a file system Physically organized into tracks and sectors Read write heads move over specified areas of the hard disks to store write or retrieve read data Random access device Can read or write data directly anywhere on the disk Faster than sequential access which reads and writes from beginning to end Makes use of the file system to organize files Connecting with Computer Science 6 Figure 10 3 Hard disk platters are divided into tracks and sectors and read write heads store and retrieve data Connecting with Computer Science 7 File Systems and Operating Systems The type of file management system is dependent on the operating system FAT file allocation table Used from MS DOS to Windows ME NTFS New Technology File System Default for Windows NT through Windows 2003 Unix and Linux support several file systems XFS JFS ReiserFS ext3 and others HFS The current Mac OS X file system Connecting with Computer Science 8 FAT Groups hard drive sectors into clusters Increases performance by organizing blocks of sectors contiguously Maintains the relationship between files and clusters being used for the file Clusters have two entries in the table Current cluster information Link to the next cluster or a special code indicating it is the last cluster Keeps track of writable clusters and bad clusters Connecting with Computer Science 9 Figure 10 4 Sectors are grouped into clusters on a hard disk Connecting with Computer Science 10 FAT continued Organizes the hard drive into Partition boot record Contains information on how to access the volume with a file system Main and backup FAT If an error occurs in reading the main FAT the backup is copied to the main to ensure stability Root directory Contains entries for every file and folder in the directory Connecting with Computer Science 11 Figure 10 5 Typical FAT file system Connecting with Computer Science 12 Defragmentation Occurs when files have clusters scattered in different locations on the storage medium rather than in a contiguous location Windows provides the Disk Defragmenter utility to reorganize clusters contiguously Improves performance by minimizing movement of the read write heads Should be used regularly to ensure system runs at peak performance Connecting with Computer Science 13 Figure 10 6 Files become fragmented as they are stored in noncontiguous clusters a defragmenting utility moves files to contiguous clusters and improves disk performance Connecting with Computer Science 14 Advantages of FAT Efficient use of disk space Does not have to use contiguous space for large files File names FAT32 can have up to 255 characters Easy to undelete files that have been deleted When a file is deleted the system places a hex value of E5h in the first position of the file name File remains on drive and can be undeleted by providing the original letter in the undelete process Connecting with Computer Science 15 Disadvantages of FAT Overall performance slows down as more files are stored on the partition Hard drive can quite easily become fragmented Lack of security NTFS provides access rights to files and directories File integrity problems Lost clusters Invalid files and directories Allocation errors Connecting with Computer Science 16 NTFS Overcomes limitations of the FAT system Is a journaling file system Keeps track of transaction performed and rolls back transactions if errors are found Uses a master file table MFT to store data about every file and directory on the volume Similar to a database table with records for each file and directory Uses clusters and reserves blocks of space to allow the MFT to grow Connecting with Computer Science 17 Advantages of NTFS File access is very fast and reliable With the MFT the system can recover from problems without losing significant amounts of data Security is greatly increased over FAT File encryption with EFS Encrypting File System and file attributes File compression Process of reducing file size to save disk space Connecting with Computer Science 18 Disadvantages of NTFS Large overhead Not recommended for volumes less than 4 GB Cannot access NTFS volumes from MS DOS Windows 5 or Windows 98 Connecting with Computer Science 19 Comparing File Systems Choosing the correct file system is operating system dependent NTFS is recommended for Windows systems Today s networked environments need security Today s machines use tools that require large volumes If the hard drive is 10 GB or less FAT is more efficient in handling smaller volumes of data UNIX Linux have many file system choices Connecting with Computer Science 20 Connecting with Computer Science 21 Connecting with Computer Science 22 Connecting with Computer Science 23 Connecting with Computer Science 24 File Organization Binary or text Binary files are computer readable but not human readable i e executable programs image files Faster to access than text files Text files consist of ASCII or Unicode characters Easy to view and modify with application programs Sequential or random access Sequential data is accessed one chunk after the other in order Random access data can be accessed in any order Connecting with Computer Science 25 Figure 10 7 Sequential vs random access Connecting with Computer Science 26 Sequential Access Starts at the beginning of the file and processes to the end of the file Writing process is very fast because new data is added to the end of a file Inserting deleting or modifying data can be very slow Can store data in rows like a