Basic Disk Data Storage in the File Systems | CIT 595, Study notes of Computer Science

Download Basic Disk Data Storage in the File Systems | CIT 595 and more Study notes Computer Science in PDF only on Docsity! 1 File System CIT 595 Spring 2008 2CIT 595 Disk Data Storage To access data in low-level format is cumbersome Need to know about tracks and sectors OS provides a uniform logical view of information storage via the File-System Known as high-level or logical formatting Logical storage unit a.k.a file File System provides method for storing and organizing files and the data they contain to make it easy to find and access them A file is nothing more than a sequence of bytes A file is stored in one or more blocks on disk A block is unit of transfer between Memory and Disk 3CIT 595 Block A block comprises of one or more sectors of the disk Depends on block allocation method used by the OS OS maintains info on which blocks belong to a file Data structures use logical block numbers Starting at 0 and running through consecutive integers up to some maximum Logical block number is converted into physical disk address i.e. Physical Disk Address -> track/cylinder, surface, sector Conversion is performed by the device driver which outputs low-level hardware specific instructions to disk controller 4CIT 595 Disk Partitioning Is the creation of logical divisions (isolated sections) of a hard disk Partitioned into one or more groups of cylinders Gives appearance of more than one hard drive e.g. C drive, D drive Disk can be prepared for use, or even dedicated to different uses Allows one to apply OS-specific logical formatting per partition Allow more than one OS per partition Encapsulate your data Corruption is local to a partition, so stand to lose only some of your data if an accident occurs Also improve disk efficiency E.g. Windows depending on the File System, assigns block sizes based on partition size Larger disk size implies larger block size that can result in wasted space 2 5CIT 595 Disk Layout Master Boot Block (MBR) A known sector on disk is used to boot the computer Partition Table Starting and Ending Addressing of each partition One of the partitions is marked as active Disk Partitions Boot Block Contains a program where the OS resides in the partition Even if no OS for that partition, a boot block is set aside Partition Control Block Magic number to denote what kind of file system e.g. FAT, NTFS, UFS Number of Blocks and the size of blocks Data Structure for Free Blocks, Directory, File Allocation Info, and the files itself 6CIT 595 Boot Block Upon boot, the BIOS (Basic Input/Output System) program is executed Is stored in ROM that gets executed on startup Conducts a basic hardware check, to determine whether all of the attachments are present and working Then executes program located in the MBR MBR program locates active partition, reads in its first block i.e. boot block and execute it Boot block loads the OS and then the OS takes over 7CIT 595 Directory Directory provides information needed to find the disk data blocks of a file A directory is nothing more than a file itself, except it is specially structured Each entry in the directory contains Symbolic file name Indicate where file is located on disk Depends on the Block Allocation Scheme for files File attributes: size, access-information, time and date Root directory contains information about where each sub- directory is located Always contained at fixed within a partition 8CIT 595 Block Allocation for files How do we allocate files on disk so that Space is utilized effectively Allow accesses that are Random/Direct: each block can be accessed independently of other Quick – try to minimize seek time Two kinds of allocation Contiguous Linked File Allocation Table (FAT) File Index