File Management in Representative Operating Systems

1. File Managementin Representative Operating Systems

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