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File SystemsLong-term Information StorageFile NamingFile StructureFile TypesFile AccessFile AttributesFile OperationsAn Example Program Using File System Calls (1/2)An Example Program Using File System Calls (2/2)Memory-Mapped FilesDirectoriesSingle-Level Directory SystemsTwo-level Directory SystemsHierarchical Directory SystemsPath NamesDirectory OperationsFile System ImplementationImplementing Files (1)Implementing Files (2)Implementing Files (3)Implementing Files (4)Implementing Directories (1)Implementing Directories (2)Shared Files (1)Shared Files (2)Disk Space Management (1)Disk Space Management (2)Disk Space Management (3)Disk Space Management (4)File System Reliability (1)File System Reliability (2)File System Reliability (3)File System Performance (1)File System Performance (2)Log-Structured File SystemsExample File Systems CD-ROM File SystemsThe CP/M File System (1)The CP/M File System (2)The MS-DOS File System (1)The MS-DOS File System (2)The Windows 98 File System (1)The Windows 98 File System (2)The Windows 98 File System (3)The UNIX V7 File System (1)The UNIX V7 File System (2)The UNIX V7 File System (3)1Chapter 6File Systems6.1 Files 6.2 Directories 6.3 File system implementation 6.4 Example file systems2Long-term Information Storage1. Must store large amounts of data2. Information stored must survive the termination of the process using it3. Multiple processes must be able to access the information concurrently3File NamingTypical file extensions.4File Structure• Three kinds of files– byte sequence– record sequence– tree5File Types(a) An executable file (b) An archive6File 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 marker7File AttributesPossible file attributes8File Operations1. Create2. Delete3. Open4. Close5. Read6. Write7. Append8. Seek9. Get attributes10.Set Attributes11.Rename9An Example Program Using File System Calls (1/2)10An Example Program Using File System Calls (2/2)11Memory-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 xyz12DirectoriesSingle-Level Directory Systems• A single level directory system– contains 4 files– owned by 3 different people, A, B, and C13Two-level Directory SystemsLetters indicate owners of the directories and files14Hierarchical Directory SystemsA hierarchical directory system15Path NamesA UNIX directory tree16Directory Operations5. Readdir6. Rename7. Link8. Unlink1. Create2. Delete3. Opendir4. Closedir17File System ImplementationA possible file system layout18Implementing Files (1)(a) Contiguous allocation of disk space for 7 files(b) State of the disk after files D and E have been removed19Implementing Files (2)Storing a file as a linked list of disk blocks20Implementing Files (3)Linked list allocation using a file allocation table in RAM21Implementing Files (4)An example i-node22Implementing Directories (1)(a) A simple directoryfixed size entriesdisk addresses and attributes in directory entry(b) Directory in which each entry just refers to an i-node23Implementing Directories (2)• Two ways of handling long file names in directory– (a) In-line– (b) In a heap24Shared Files (1)File system containing a shared file25Shared Files (2)(a) Situation prior to linking(b) After the link is created(c)After the original owner removes the file26Disk Space Management (1)Block size• Dark line (left hand scale) gives data rate of a disk• Dotted line (right hand scale) gives disk space efficiency• All files 2KB27Disk Space Management (2)(a) Storing the free list on a linked list(b) A bit map28Disk 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 blocks29Disk Space Management (4)Quotas for keeping track of each user’s disk use30File System Reliability (1)File that hasnot changed• 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 number31File System Reliability (2)Bit maps used by the logical dumping algorithm32File System Reliability (3)• File system states(a) consistent(b) missing block(c) duplicate block in free list(d) duplicate data block33File System Performance (1)The block cache data structures34File System Performance (2)• I-nodes placed at the start of the disk• Disk divided into cylinder groups– each with its own blocks and i-nodes35Log-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 blocks36Example File SystemsCD-ROM File SystemsThe ISO 9660 directory entry37The CP/M File System (1)Memory layout of CP/M38The CP/M File System (2)The CP/M directory entry format39The MS-DOS File System (1)The MS-DOS directory entry40The MS-DOS File System (2)• Maximum partition for different block sizes• The empty boxes represent forbidden combinations41The Windows 98 File System (1)BytesThe extended MOS-DOS directory entry used in Windows 9842The Windows 98 File System (2)BytesChecksumAn entry for (part of) a long file name in Windows 9843The Windows 98 File System (3)An example of how a long name is stored in Windows 9844The UNIX V7 File System (1)A UNIX V7 directory entry45The UNIX V7 File System (2)A UNIX i-node46The UNIX V7 File System (3)The steps in looking up


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UCSC CMPS 111 - File Systems

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