Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 11: File System Implementation

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter 11: File System Implementation File System Structure File System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery Log-Structured File Systems

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File-System Structure File structure Logical storage unit Collection of related information File system resides on secondary storage (disks) File system organized into layers File control block – storage structure consisting of information about a file

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Layered File System

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Layered File System Summary I/O control – device drivers & interrupt handlers Basic File System – issues command to device driver to read/write physical blocks (uses numeric disk address: drive 1, cyl 73, track 2, sector 10) File-Organization Module – translates logical block address to physical block address, manages free-space Logical File System – Manages the “metadata” – information relating to the file-system structure via FCBs

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Overview On- Disk structures Boot control block – information needed to boot the system Partition control block – contains partition details – number of blocks in partition, size of block, free- block data Directory structure – contains data about organization of files FCBs – actual file data In-Memory structures Partition table – information about mounted partitions Directory structure – recently accessed directories System-wide open-file table – FCB of open file Per-process open-file table – pointer to system-wide open-file table’s entry … (index is returned from open( ))

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Typical File Control Block

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition In-Memory File System Structures The following figure illustrates the necessary file system structures provided by the operating systems. Figure 12-3(a) refers to opening a file. Figure 12-3(b) refers to reading a file.

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition In-Memory File System Structures

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Directory Entry

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Example of a File lookup

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Directory Implementation Linear list of file names with pointer to the data blocks. simple to program time-consuming to execute Hash Table – linear list with hash data structure. decreases directory search time collisions – situations where two file names hash to the same location fixed size – difficult to change size of directory- must change hash function and reorganize all entries

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation Each file occupies a set of contiguous blocks on the disk. Advantages Simple – only starting location (block #) and length (number of blocks) are required. Entire file can be read from disk in one operation Random access. Disadvantages Need to know maximum size at creation Extreme fragmentation until reaches fullsize Files cannot grow – size may be underestimated Placement strategies – first fit/best fit

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation of Disk Space

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Extent-Based Systems Many newer file systems (I.e. Veritas File System) use a modified contiguous allocation scheme. Extent-based file systems allocate disk blocks in extents. An extent is a contiguous block of disks. Extents are allocated for file allocation. A file consists of one or more extents.

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. pointer block =

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation (Cont.) Advantages Simple – need only starting address Free-space management system – no waste of space Fragmentation only in last block Disadvantages No random access If one block goes bad – lose entire file Pointer is substantial overhead Alternative: collect blocks into clusters and allocate clusters – pointers use much smaller percentage of disk space

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked using Index Remove pointer from data block and store pointer in a table Advantages Speed up random access Lookup can be done in memory Disadvantages Tables needs to be in memory Small blocks create large tables Example: File Allocation Table – Index set aside at start of disk partition. Indexed by physical blocks (0 indicates free block).

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File-Allocation Table

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation Brings all pointers together into the index block. The ith index indicates physical address of ith block Logical view. index table

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Example of Indexed Allocation

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation (Cont.) Advantages Random access is simple and fast Files can grow – add entry to index Disadvantages Index table has fragmentation Random access How large should the index block be? How should we index large files? Linked scheme – link several indexes together Multilevel index – use first level to point to next level Combine – UNIX inode scheme

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Indexed Allocation – Multilevel  outer-index index table file

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Combined Scheme: UNIX

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management Bit vector (n blocks) … 012n-1 bit[i] =  0  block[i] free 1  block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) Bit map requires extra space. Example: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes) Easy to get contiguous files

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) Linked list (free list) - free list head pointer points to first free block. Cannot get contiguous space easily No waste of space – only one pointer to keep in memory Allocation – remove block on front of list, change head pointer to next block Adding free block – add to front of list

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Free Space List on Disk

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) Grouping – Use first free block to store the next n free block addresses. At end of this block point to n+1 block which contains the next n free blocks. Only one block needs to be stored in memory. Allocation – delete first entry in first block Adding to free list – add entry to first available spot in block (may need to go to next block) Counting – If blocks are allocated contiguously – store address of start block and number of free blocks in the same format as grouping

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) Need to protect: Pointer to free list Bit map  Must be kept on disk  Copy in memory and disk may differ.  Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk. Solution:  Set bit[i] = 1 in disk.  Allocate block[i]  Set bit[i] = 1 in memory

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Efficiency and Performance Efficiency dependent on: disk allocation and directory algorithms types of data kept in file’s directory entry Performance disk cache – separate section of main memory for frequently used blocks free-behind and read-ahead – techniques to optimize sequential access improve PC performance by dedicating section of memory as virtual disk, or RAM disk.

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Recovery Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies. Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape). Recover lost file or disk by restoring data from backup.

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 11