MOD 2 - File Systems and Disks.pdf

File Systems and Disk Management Learning Objectives 2.1 Files 2.2 Directories 2.3 File system implementation 2.4 Example file systems MOD 1 – INTRODUCTION TO SYSTEMS ADMINISTRATION 2 Long-term Information Storage 1. Must store large amounts of data 2. Information stor...

File Systems and Disk Management Learning Objectives 2.1 Files 2.2 Directories 2.3 File system implementation 2.4 Example file systems MOD 1 – INTRODUCTION TO SYSTEMS ADMINISTRATION 2 Long-term Information Storage 1. Must store large amounts of data 2. Information stored must survive the termination of the process using it 3. Multiple processes must be able to access the information concurrently 3 File Naming Typical file extensions. 4 File Structure Three kinds of files byte sequence record sequence tree 5 File Types (a) An executable file (b) An archive 6 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 7 File Attributes Possible file attributes 8 File Operations 1. Create 7. Append 2. Delete 8. Seek 3. Open 9. Get attributes 4. Close 10.Set Attributes 5. Read 11.Rename 6. Write 9 An Example Program Using File System Calls (1/2) 10 An Example Program Using File System Calls (2/2) 11 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 12 Directories Single-Level Directory Systems A single level directory system contains 4 files owned by 3 different people, A, B, and C 13 Two-level Directory Systems Letters indicate owners of the directories and files 14 Hierarchical Directory Systems A hierarchical directory system 15 Path Names A UNIX directory tree 16 Directory Operations 1. Create 5. Readdir 2. Delete 6. Rename 3. Opendir 7. Link 4. Closedir 8. Unlink 17 File System Implementation A possible file system layout 18  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 19 Implementing Files (2) Storing a file as a linked list of disk blocks 20 Implementing Files (3) Linked list allocation using a file allocation table in RAM 21 Implementing Files (4) An example i-node 22  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 23 Implementing Directories (2) Two ways of handling long file names in directory (a) In-line (b) In a heap 24 Shared Files (1) File system containing a shared file 25 Shared Files (2) (a) Situation prior to linking (b) After the link is created (c)After the original owner removes the file 26  Disk 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 2KB 27 Disk Space Management (2) (a) Storing the free list on a linked list (b) A bit map 28  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 29 Disk Space Management (4) Quotas for keeping track of each user’s disk use 30 File System Reliability (1) File that has not 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 number 31 File System Reliability (2) Bit maps used by the logical dumping algorithm 32 File System Reliability (3) File system states (a) consistent (b) missing block (c) duplicate block in free list (d) duplicate data block 33 File System Performance (1) The block cache data structures 34 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 35  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 36 Example File Systems CD-ROM File Systems The ISO 9660 directory entry 37 The CP/M File System (1) Memory layout of CP/M 38 The CP/M File System (2) The CP/M directory entry format 39 The MS-DOS File System (1) The MS-DOS directory entry 40 The MS-DOS File System (2) Maximum partition for different block sizes The empty boxes represent forbidden combinations 41  The Windows 98 File System (1) Bytes The extended MS-DOS directory entry used in Windows 98 42  The Windows 98 File System (2) Bytes Checksum An entry for (part of) a long file name in Windows 98 43 The Windows 98 File System (3) An example of how a long name is stored in Windows 98 44 The UNIX V7 File System (1) A UNIX V7 directory entry 45 The UNIX V7 File System (2) A UNIX i-node 46 The UNIX V7 File System (3) The steps in looking up /usr/ast/mbox 47  ***END*** MOD 1 – INTRODUCTION TO SYSTEMS ADMINISTRATION 48