1 File Management in Representative Operating Systems

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 2 UNIX File Management File –Stream of bytes –Internal logical structure is application specific Types of files –Ordinary Contains information entered by user, application program, or system utility program –Directory Ordinary file with special protection privileges (file system can write, user programs can read) Contains a list of file names and pointers to the associated index or information nodes (inodes) –Special Used for I/O devices, each device is associated with a special file –Named Named pipe

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 3 UNIX File Management (cont.) Inodes –Control structure that contains information needed by the OS for a particular file –Contents File mode: File type, execution flags, access permissions (RWE for OGO) Link count: Number of directory references to this node Owner ID: Owner of file Group ID: Group owner File size: Number of bytes in file File address: 39 bytes of address information Last accessed: Time of last file access Last modified: Time of last file modification Inode Modified: Time of last inode modification

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 4 UNIX File Management (cont.) File allocation on secondary storage –Dynamic allocation, on a block basis, non-contiguous –Indexed method of addressing, with part of the index stored in file’s inode –Inode has 39 bytes of address information, structured as thirteen 3-byte pointers (addresses) First ten addresses point to the first 10 data blocks Eleventh address points to a block on disk that contains the next stage of the index (single indirect block) Twelfth address points to a double indirect block Thirteenth address points to a triple indirect block –Total number of blocks depends on the capacity of the fixed-size blocks in the system (e.g., in UNIX System V each block has 1Kbyte and can hold 256 block addresses. Total size of a file is 16Gbytes.) –Advantages The inode has a fixed size, is small, and can be kept be in main memory Small files can be accessed with no indirection The maximum size can accommodate any application

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 5 UNIX File Management (cont.)

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 6 Windows 2000 File Management File systems supported –File Allocation Table (FAT) from Windows 95, MS-DOS, and OS/2 –W2K File System (NTFS) NTFS key features –Recoverability Uses transaction processing model to reconstruct disk volumes after failures Uses redundant storage for critical system data –Security Security descriptor for each file object defines security attributes –Large disks and large files Supports very large disks and files –Multiple data streams Multiple data streams can be defined for a file –General indexing facility A file can be indexed by any of the attributes associated with the file

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 7 Windows 2000 File Management (cont.) NTFS volume and file structure –Disk storage units Sector: Smallest physical storage unit on disk; power of 2 in bytes, usually 512 bytes Cluster: One or more contiguous sectors; size in sectors is power of 2 Volume: –Logical partition on disk used by file system to allocate storage, one or more clusters –Consists of file system information, a collection of files, and any remaining unallocated space on the volume –Can be a portion or an entire single disk, or can extend across multiple disks –Maximum size: 2e64 –Cluster is the fundamental unit of allocation System is configured initially with a certain number of sectors per cluster Use of clusters allows the use of non-standard disks, with sectors different from 512 bytes Maximum file size is 2e32, which represents 2e48 bytes

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 8 Windows 2000 File Management (cont.) NTFS volume layout –Every element on a volume is a file and every file consists of a collection of attributes –Regions on the NTFS volume Partition boot sector –Information about the volume layout and the file system structures –Boot startup information and code Master File Table (MFT) –Information about all the files and folders (directories) on this volume –Information about available unallocated space –Organized as a table of variable-length rows, called records –Each row describes a file or folder on this MFT and contains »Attributes for the file or folder (e.g., file name, access rights, time stamps) »Pointers to clusters containing file System files region –MFT2: duplicate of the first three rows of MFT –Log file: list of transaction steps for file recoverability –Cluster bit map: volume representation, showing which clusters are in use –Attribute definition table: defines the attribute types supported on this volume

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 9 Windows 2000 File Management (cont.) Recoverability –The file system can be recovered to a consistent state following a system crash or disk failure –Elements that support recoverability I/O manager –NTFS driver: provides the basic open, close, read, write functions –Fault-tolerant driver (FTDISK) provides the software RAID functionality Log file service –Maintains a log of disk writes –Used to recover volume after a system failure Cache manager –Caches file reads and writes for improved performance (lazy write and lazy commit) Virtual memory manager –Cached file are accessed by mapping file references to virtual memory references and reading and writing virtual memory –Logging is the main technique used to implement recoverability Each operation that changes the file system is treated as a transaction Transactions are recorded in a log file Partially completed transactions can be redone or undone after recovery

CS-550 (M.Soneru): File Management in Representative Operating Systems: [Sta’01] 10 Windows 2000 File Management (cont.)