Bài giảng Hệ điều hành - Chapter 6: File Systems

File Systems Chapter 6 6.1 Files 6.2 Directories 6.3 File system implementation 6.4 Example file systems Long-term Information Storage Must store large amounts of data Information stored must survive the termination of the process using it Multiple processes must be able to access the information concurrently File Naming Typical file extensions. File Structure Three kinds of files byte sequence record sequence tree File Types (a) An executable file (b) An archive File Access Sequential access read all bytes/records from the beginning cannot jump around, could rewind or back up convenient when medium was mag tape Random access bytes/records read in any order essential for data base systems read can be … move file marker (seek), then read or … read and then move file marker File Attributes Possible file attributes File Operations Create Delete Open Close Read Write Append Seek Get attributes Set Attributes Rename An Example Program Using File System Calls (1/2) An Example Program Using File System Calls (2/2) Memory-Mapped Files (a) Segmented process before mapping files into its address space (b) Process after mapping existing file abc into one segment creating new segment for xyz DirectoriesSingle-Level Directory Systems A single level directory system contains 4 files owned by 3 different people, A, B, and C Two-level Directory Systems Letters indicate owners of the directories and files Hierarchical Directory Systems A hierarchical directory system A UNIX directory tree Path Names Directory Operations Create Delete Opendir Closedir Readdir Rename Link Unlink File System Implementation A possible file system layout Implementing Files (1) (a) Contiguous allocation of disk space for 7 files (b) State of the disk after files D and E have been removed Implementing Files (2) Storing a file as a linked list of disk blocks Implementing Files (3) Linked list allocation using a file allocation table in RAM Implementing Files (4) An example i-node Implementing Directories (1) (a) A simple directory fixed size entries disk addresses and attributes in directory entry (b) Directory in which each entry just refers to an i-node Implementing Directories (2) Two ways of handling long file names in directory (a) In-line (b) In a heap Shared Files (1) File system containing a shared file Shared Files (2) (a) Situation prior to linking (b) After the link is created (c)After the original owner removes the file Disk Space Management (1) Dark line (left hand scale) gives data rate of a disk Dotted line (right hand scale) gives disk space efficiency All files 2KB Block size Disk Space Management (2) (a) Storing the free list on a linked list (b) A bit map Disk Space Management (3) (a) Almost-full block of pointers to free disk blocks in RAM - three blocks of pointers on disk (b) Result of freeing a 3-block file (c) Alternative strategy for handling 3 free blocks - shaded entries are pointers to free disk blocks Disk Space Management (4) Quotas for keeping track of each user’s disk use File System Reliability (1) A file system to be dumped squares are directories, circles are files shaded items, modified since last dump each directory & file labeled by i-node number File that has not changed File System Reliability (2) Bit maps used by the logical dumping algorithm File System Reliability (3) File system states (a) consistent (b) missing block (c) duplicate block in free list (d) duplicate data block File System Performance (1) The block cache data structures File System Performance (2) I-nodes placed at the start of the disk Disk divided into cylinder groups each with its own blocks and i-nodes Log-Structured File Systems With CPUs faster, memory larger disk caches can also be larger increasing number of read requests can come from cache thus, most disk accesses will be writes LFS Strategy structures entire disk as a log have all writes initially buffered in memory periodically write these to the end of the disk log when file opened, locate i-node, then find blocks Example File Systems CD-ROM File Systems The ISO 9660 directory entry The CP/M File System (1) Memory layout of CP/M The CP/M File System (2) The CP/M directory entry format The MS-DOS File System (1) The MS-DOS directory entry The MS-DOS File System (2) Maximum partition for different block sizes The empty boxes represent forbidden combinations The Windows 98 File System (1) The extended MOS-DOS	 directory entry used in Windows 98 Bytes The Windows 98 File System (2) An entry for (part of) a long file name in Windows 98 Bytes Checksum The Windows 98 File System (3) An example of how a long name is stored in Windows 98 The UNIX V7 File System (1) A UNIX V7 directory entry The UNIX V7 File System (2) A UNIX i-node The UNIX V7 File System (3) The steps in looking up /usr/ast/mbox