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File System Implementation. Ansu Na(asna@archi.snu.ac.kr) School of Computer Science and Engineering Seoul National University. Introduction. There are various file systems They have different data structures Each of them has its own merits or demerits
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File System Implementation Ansu Na(asna@archi.snu.ac.kr) School of Computer Science and Engineering Seoul National University
Introduction • There are various file systems • They have different data structures • Each of them has its own merits or demerits • We will look into the vsfs (Very Simple Files System) • It introduces most concepts of file systems • Two important aspects of file systems • Data structures • Access methods • Two methods for the performance improvement • Caching • Buffering
Overall Organization S : Super block I : Inode table D : Data block i : Inode bitmap d : Data bitmap • Divide the disk into blocks with same size (4KB) • Assume the disk has 64 blocks (256KB) • Classify some portions of blocks for specific usages • The data region for user data (56 blocks) • The inode table for inodes which have the key metadata (5 blocks) • The inode bitmap and the data bitmap as the allocation structures (2 blocks) • Each bit indicate an object free (0) or in-use (1) • The superblock for overall information of file system
Inode 0KB 4KB 8KB 12KB 16KB 20KB 24KB 28KB 32KB • The position of inode can be calculated • blk = (inumber * sizeof(inode_t)) / blockSize; • sector = ((blk * blockSize) + inodeStartAddr) / sectorSize; • An inode holds the metadata for a file • Length, permissions, location of its constituent blocks • An inode is referred to by a inumber • Low level name of the file Example> 20KB inode table ( 5 * 4KB block )
Directory • A special type of file • Represents hierarchical structure of files • Is a list of file names with inode number • Stored in data blocks pointed to by inodes Example) A directory dir which has three files in it Inode Number
Free Space Management Inode Bitmap Inode Table Data Bitmap Data Region • To allocate a new file or directory • Tracks which inodes and data blocks are free • Uses inode bitmap and data bitmap here • Other methods such as free lists or B-tree are possible • Provide pre-allocation policy for allocating a new file • Look for a sequence of free blocks and allocate them to new file • Improve performance by placing a portion of file sequentially on disk
Reading A File From Disk Proportional to the length of the pathname A file system must know the inumber of root when it is mounted Access the inode of root to find the data block Do not access allocation structures on read operation Find the inumber of foo Access the inode of foo to find the data block Find the inumber of bar Allocate a file descriptor in open-file table Access the inode of bar to find the first data block Read first data block Update access time • An assumption • Only superblock is in memory • Steps for reading a file • open() / read() / close() • An example • Read /foo/bar
Writing to Disk Access the inode of root to find the data block Find the inumber of foo Access the inode of foo to find the data block Read the directory data for update Read the inode bitmap and find a free entry in inode table Update the allocation information Write the directory data including information of bar Read the inode data for update Write the inode data with proper values Update modify time Read the inode of bar for update Read the data bitmap and find a freeedata black Update the allocation information Write the user data onto data block Write the inode data with proper values An example : writing on /foo/bar which is a new file
Caching and Buffering The write operations can be scheduled in sequential order The number of I/O operation can be reduced from 8 to 2 If the file is deleted before any write operation, entire updates can be avoided • Use system memory to cache important blocks • A fix-sized cache or unified page cache • Write buffering for batching or scheduling or avoiding updates
Summary • Inodes store meta data of files • Directories provide hierarchical structure with name-> inumber mappings • Bitmaps can be used for allocation structures • Only A Read or a write can cause a lot of I/O operations • Traversing inodes and updating information are occurred • Caching and write buffering can be solutions • Files system design is very flexible • A lot of structures and methods can be used
