FILE SYSTEM IMPLEMENTATION Sunu Wibirama
File-System Structure Outline File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
File-System Structure Outline File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
File System Structure File system is provided by OS to allow the data to be stored, located, and retrieved easily Two problems on file system design: How the file system should look to the user Algorithms and data structures to map logical file system onto the physical secondary-storage devices File system organized into layers, uses features from lower levels to create new features for use by higher levels. I/O Control controls the physical device using device driver Basic file system needs only to issue generic commands to the device driver to read and write physical block on the disk (ex. drive 1, cylinder 73, track 2, sector 11)
File-System Structure Outline File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
File System Implementation File organization module knows physical and logical blocks, translating logical block address to physical block address. It also manages free-space on the disk Logical file system manages metadata information (all file system structure except the actual data or contents of the file). Logical file system maintains file structure via file-control blocks (FCB) File control block storage structure consisting of information about a file Basic file system and I/O control can be used by multiple file systems. Recently, OS supports more than one FS
A Typical File Control Block
On-Disk File System Structures Boot control block contains info needed by system to boot OS from that volume (UNIX: boot block, NTFS: partition boot sector) Volume control block contains volume details (UNIX: superblock, NTFS: master file table) Directory structure organizes the files (UNIX: inode numbers, NTFS: master file table) Per-file File Control Block (FCB) contains many details about the file
In-Memory File System Structures It is used for file-system management and performance improvement (via caching). The data are loaded at mount time and discarded at dismount. The structures including: In-memory mount table: information of each mounted volume In-memory directory structure System-wide open-file table: a copy of FCB of each open file Per-process open-file table: a pointer to the appropriate entry in the system-wide open-file table, as well as other information based on process that uses the file.
New File Creation Process Application program calls the logical file system Logical file system knows the directory structures. It allocates a new FCB. The system then reads the appropriate directory into memory, updates it with the new file name and FCB, and writes it back to the disk. Now, the new created file can be used for I/O operation, which will be explained in the next slide
In-Memory File System Structures File open File read
File-System Structure Outline File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
Directory Implementation Directory-allocation and directory-management algorithms significantly affects the efficiency, performance, and reliability of the file system. There are two impementations: linear list and hash table Linear list of file names with pointer to the data blocks. simple to program time-consuming to execute, because it requires a linear search to create or delete file. Hash Table linear list with hash data structure. decreases directory search time problem: collisions (two file names hash to the same location) fixed size of hash table (for 64 entries, the hash function converts filename into integers from 0 to 63)
File-System Structure Outline File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
Review: Dynamic Storage Allocation and Fragmentation
Dynamic Storage Allocation At any given time, we have a set of free spaces (holes) of various sizes scattered through the disk. Dynamic Storage Allocation problem: satisfying a request of size n from a list of free spaces There are three solutions for dynamic storage allocation problem: First Fit: allocate the first hole that is big enough. Stop searching the entire list as soon as we find a free hole that is large enough Best Fit: Allocate the smallest hole that is big enough. We must search the entire list in the storage Worst Fit: allocate the largest hole. Again, we must search the entire list in the storage In term of speed and disk utilization : first fit and best fit are better then worst fit
External Fragmentation OS 50k 125k File 9? File 3 File 8 100k File 2 First fit and best fit suffer from external fragmentation : there is enough total storage space to satisfy the request, but the available spaces are not contiguous
Internal Fragmentation A (part 1) Allocated to A A (part 2) A (part 3) OS Room for growth Another approach : break the physical storage into fixed-sized blocks and allocate storage in units based on block size. The storage space allocated to file may be slightly larger than the requested space, but we get internal fragmentation.
Fragmentation Solution? Minimize external fragmentation? Large disk blocks (but internal fragmentation occurs) Choose the best allocation methods to allocate disk block Compaction: Shuffle memory contents to place all free memory together in one large block (example: disk defragmenter) Only for execution time address binding data (dynamic address relocation) (a) (b) OS 50k OS File 3 OS 125k File 9 File 3 90k File 8 File 8 60k File 8 100k File 3 File 2 File 2 File 2
Next, we want to allocate disk blocks effectively. But how?
Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation
Contiguous Allocation of Disk Space
OS : IBM VM/CMS Contiguous Allocation Each file occupies a set of contiguous blocks on the disk Simple only starting location (block #) and length (number of blocks) are required Both sequential and direct access are supported Disadvantages: Wasteful of space (dynamic storage-allocation problem) External fragmentation: free space is broken into chunks We must determine the size of needed space before we allocated Preallocation may be inefficient if the file grow slowly over a long period (months or years). The file therefore has a large amount of internal fragmentation
Contiguous Allocation (a) Contiguous allocation of disk space for 7 files. (b) The state of the disk after files D and F have been removed.
Contiguous Allocation One of several solutions: use a modified contiguous allocation scheme (ex. : Veritas file system, replacing standard UNIX UFS) Initially, a contiguous block of space is allocated If it is not to be large enough, another block is added, which is called extent. An extent is a contiguous block of disks A file consists of one or more extents Another approach, we use Linked Allocation Method
Linked Allocation Linked allocation solves all problems of contiguous allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. Ex: File Jeep Start at block 9 Then: block 16, 1, 10 Finally end at block 25 pointer (4 bytes) 1 block (512 bytes) 508 bytes visible part to user
Linked Allocation Advantages: Free-space management system no waste of space File can grow, depends on available free blocks Disadvantages: No random access (only sequential access) Space required for pointers (0.78 percent of the disk is being used for pointers, rather than for information). Solution, uses clusters (unit of blocks), so that pointers use much smaller percentage Clusters operation increase internal fragmentation Reliability, pointer damage will cause unlinked blocks in a file. FAT (File Allocation Table): variation on linked allocation (ex.: MS-DOS and OS/2 operating systems) Located at the beginning of each volume
File-Allocation Table To do random access: 1. The disk head move to the start of volume to read the FAT 2. Find the location of the desired block 3. Move to the location of the block itself
Indexed Allocation Linked allocation: solves external fragmentation and size-declaration problem FAT + Linked Allocation = efficient file retrieval using random access Without FAT, linked allocation cannot support efficient direct access: because the pointers to the blocks are scattered with the block themselves all over the disk the pointers must be retrieved in order (sequential access). How about collecting all pointers together into one location?
Indexed Allocation Brings all pointers together into the index block Each file has its own index block, which is an array of disk-block addresses Directory contains the address of index block Support direct access without external fragmentation Each file has its allocation for all pointers, so that it has wasted space greater than linked allocation (which contains one pointer per block). We want the index block as small as possible, then we have several mechanisms: 1. Linked scheme 2. Multilevel index 3. Combined scheme Index block
Indexed Allocation Linked Scheme An index block is normally one disk block Large files -> we can link together several index blocks Example: an index block contains: - a small header of file name - a set of 100 disk-block addresses - nil (for small file) or a pointer to another index block (for a large file) Multilevel index First-level index block points to second-level index blocks which in turn point to the file blocks (see next slide) Combined scheme In unix, for example: 15 pointers in fileʼs inode 12 first pointer: direct blocks, for small file (no more than 12 blocks). If the block size is 4KB, then up to 48KB (12 x 4KB) can be accessed directly The next pointers point to indirect blocks, which implement multilevel index based on their sequence (see next two slide)
Multilevel Index Back to Indexed Allocation 1st-level index block 2nd-level index block file
Combined Scheme: UNIX UFS (4K bytes per block) Back to Indexed Allocation
File-System Structure Outline File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
Free-Space Management Disk space is limited, we need to reuse the space from deleted files for new files, if possible To keep track of free disk space, the system maintains a free-space list. Free-space list records all free disk blocks: those not allocated to some file or directory Creating file: search the free-space list for the required amount of space allocate that space to the new file remove that space from the free-space list Deleting file: the disk space of the file is added to free-space list
Free-Space Management Free-space list concept Bit vector (n blocks) Bit vector requires extra space. Example: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n (amount of bits) = 2 30 /2 12 = 2 18 bits (or 32K bytes) 0 1 2 n-1 bit[i] = 0 block[i] free 1 block[i] occupied Finding the free block number {(number of bits per word) *(number of 0-value words)} +offset of first 1 bit 0 1 2 3 4 5 6 7 8 9 10 11 0 0 1 1 1 1 0 0 0 1 1 0 1st word 2nd Word 3rd word Assume 1word = 3bits, then finding block 11: (3 X 3)+2
Free-Space Management The other free-space management methods include: (1) Linked list Link together all the free disk blocks Keeping a pointer to the first free block in a special location on the disk and caching it in memory. The first block contains a pointer to the next free disk block. Must read each block to traverse list, increase I/O operation time. (2) Grouping Storing the address of n free blocks in the first free block. n-1 blocks are actually free blocks but the last block contains the addresses of another n free blocks. (3) Counting (for contiguous free block) Several contiguous blocks may be freed simultaneously Keep the address of the first free block and n of free contiguous blocks that follow the first block. Each entry in free-space list consists of disk address and a count (ex. address 4 + 15 --> we have 16 free blocks. 1st block starts at address 4, followed by 15 free blocks)
::Discussion
Comparing File System Fragmentation in Windows (FAT 32) and Linux (ext)
Defragmentation http://en.wikipedia.org/wiki/defragmentation
Why Linux rarely needs defragmentation tools? Does fragmentation occur in Linux? Yes, but in very small quantity Block Groups: group file-data together in clumps to manage small and large file (remember combined scheme in indexed allocation) Only write files to unused portion of the disk that are not predictably being fragmented in shorter time.
Combined Scheme Possible to allocate bigger blocks for a file
Unix System Keep fragmentation level below 20% More than 20%? You certainly need to fix your hard disk using shake-fs Run : e2fsck -nv /dev/sda1 as root, resulting: Fragmented Part
Implementation (*you should have known this before...) http://geekblog.oneandoneis2.org/index.php/2006/08/17/ why_doesn_t_linux_need_defragmenting Empty hard disk (*simplified assumption)
Start with FAT file system... I have hello.txt OK, now add bye.txt? I want to change hello.txt, dude...
1st approach Just copy, delete the original content, and wrap it up in the larger space... Or, Put your extended file content to the next space... 2nd approach If the first approach requires huge read and write operation, then the most possible approach is the second one. That s why FAT suffers from large fragmentation
What About Linux? Initial condition
What About Linux? Add bye.txt
What About Linux? Change hello.txt
Now you know... why people (again) prefer Unix based OS for handling large data Defragmentation = decreasing productivity
Thank You