File System Implementation File System Structure File System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance
Layered File System in OS
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.
A Typical File Control Block (FCB)
Logical file system
Provides users the view of a contiguous sequence of words, bytes stored somewhere. Uses a directory structure, symbolic name Provides protection and security OS/user interface E.g., to create a new file the API provides a call that calls the logical file system
The file organization module
Knows about files and their logical blocks (say 1,..N) Files are organized in blocks of 32 bytes to 4K bytes Translates logical blocks into physical Knows location of file, file allocation type Includes a free space manages that tracks unallocated blocks
Basic file system
Issues commands to the device driver (layer of software that directly controls disk hardware) to read and write physical blocks on the disk, Each physical block identified by a disk address (e.g., drive 2, cylinder 34, track 2, sector 11)
IO control
The lowest level in the file system Consists of device drivers and interrupt handlers to transfer information between the memory and the disk A device driver translates commands such as “get me block 111” into hardware specific ISA used by hardware controller. This is accomplished by writing specific bits into IO registers
Virtual File Systems
All file operations are done on an Open File Table (OFT) Contains information about all the open files Some OS can have multiple such tables, e.g., one for each process and one system-wide table After opening a file the OS updates this OFT table and returns an index to the raw in the table All future references to the file are done through the index and not the symbolic file name Index is called FCB, file descriptor (Unix) or file handle (NT) FCB effectively is another table containing info about a file for the OS Directory info generally is copied into FCB after file opening (compare this with the PCB)
FCB (is an index into a row in OFT)
Schematic View of Virtual File System
VFS allows the same system call interface (the API) to be used for different types of file systems.
The API is to the VFS interface, rather than any specific type of file system.
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Directory Implementation Has a large effect on the file system performance A) Linear list of file names with pointer to the data blocks. simplest to program time-consuming to execute as require a linear search E.g., to create a new file name we must search the directory to make sure that no such file exist When we open the file we must search the linear structure Many OS implement a software cache for the most frequently/recently accessed directory information
Allocation Methods Many files are stored on the same disk; one main problem is to allocate space on disk to the files
An allocation method refers to how disk blocks are allocated for files Three major techniques
1) Contiguous allocation Allocation for a file is contiguous on disk 2) Linked allocation Blocks are linked together on disk so can be anywhere 3) Indexed allocation Blocks are mapped with a table that contains pointers to disk blocks (similar to page table)
B) Hash Table – linear list with hash data structure.
decreases directory search time greatly collisions – situations where two file names hash to the same location Major difficulty is its fixed size and dependence of hash function on the size of the table Typical solution is to make each hash entry a linked list
Contiguous Allocation
Contiguous Allocation of Disk Space
Each file occupies a set of contiguous blocks on the disk. Simple – only starting location (block #) and length (number of blocks) are required. Random access. Wasteful of space (dynamic storage-allocation problem). External fragmentation First fit and best fit strategies to allocate
Files cannot grow.
Extent-Based Systems Many newer file systems (e.g. Veritas File System) use a modified contiguous allocation scheme. Extent-based file systems allocate disk blocks in extents.
Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. block
=
pointer
An extent is a contiguous block of disks. Extents are allocated for file allocation. A file consists of one or more extents.
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Linked Allocation (Cont.)
Linked Allocation
Simple – need only starting address Free-space management system – no waste of space No random access
File-Allocation Table (FAT) File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2. FAT table on each partition contains map of linked list
Indexed Allocation Linked allocation solves external fragmentation but cannot support direct access (FAT helps in that) Indexed allocation solves the direct access problem by
Bringing all pointers together into the index block. Index block is an array of block addresses To read the k-th block we first find its address in the index block by looking the k-th pointer
Logical view.
index table
Example of Indexed Allocation
Indexed Allocation – Mapping (Cont.)
Μ
outer-index
index table
file
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Combined Scheme: UNIX inode (4K bytes per block)
Free-Space Management Space represented with Bit vector (n blocks) 0 1
2
n-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
Free-Space Management (Cont.) Bit map requires extra space. Example: block size = 212 bytes (4 kbytes) disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes)
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 (coherence issue). 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
Easy to get contiguous files
Linked list (free list of blocks is linked together)
Cannot get contiguous space easily No waste of space
File system Efficiency and Performance
Linked Free Space List on Disk
Efficiency dependent on: disk allocation and directory algorithms
Performance
Disk/buffer cache – separate section of main memory for frequently used file blocks (regular IO) Page cache – caches pages rather than blocks with VM techniques (memory-mapped IO) Dedicating section of memory as virtual disk, or RAM disk.
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Various Disk-Caching Locations
Page Cache A page cache caches pages rather than disk blocks using virtual memory techniques. Memory-mapped I/O uses a page cache. Routine I/O through the file system uses the buffer (disk) cache. This leads to the following figure.
