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Review – I/O subsystems. OS responsibilities to device drivers Device API, buffering, naming, interrupt handling Installing device drivers Static vs. dynamic Discovering devices that are present Polling and identification Bitmap display device Bitblt to “paint” characters on screen
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Review – I/O subsystems • OS responsibilities to device drivers • Device API, buffering, naming, interrupt handling • Installing device drivers • Static vs. dynamic • Discovering devices that are present • Polling and identification • Bitmap display device • Bitblt to “paint” characters on screen • Window management File Systems
Network I/O & Project #3 • Socket abstraction • An I/O stream to a network connection • Connection: • A serial conversation between processes on separate machines • Separate from all other connections • Project #3: simple web server • Listen on a port; open a socket; make a connection File Systems
Review – Disks • Implementer of File abstraction • Storage of large amounts of data for very long times • Persistence, reliability • Controlled like I/O devices, but integral part of information storage subsystem • Rapidly increasing capacities, dropping prices • $0.5–$6.0 per gigabyte • Slowly improving transfer rates, seek performance • Only a factor of 5-10 in three decades! File Systems
Review – Disks (continued) • Organized into cylinders, tracks, sectors • Random access • Any sector can be read & written independently of any other • Very high bandwidth for consecutive reads or writes • Seek time is (often) dominating factor in performance • Bad blocks are a fact of life • Most detected during formatting • Some occur during operation • Controller or OS must step around them • Seek optimization algorithms • Popular study topics, less popular in real systems • Long seek queues system is out of balance File Systems
Files and File Systems CS-3013 & CS-502Operating Systems File Systems
File(an abstraction) • A (potentially) large amount of information or data that lives a (potentially) very long time • Often much larger than the memory of the computer • Often much longer than any computation • Sometimes longer than life of machine itself • (Usually) organized as a linear array of bytes or blocks • Internal structure is imposed by application • (Occasionally) blocks may be variable length • (Often) requiring concurrent access by multiple processes • Even by processes on different machines! File Systems
File Systems and Disks • User view • File is a named,persistent collection of data • OS & file system view • File is collection of disk blocks • File System maps file names and offsets to disk blocks • Outline for this section • Files • Directories • Implementation of Files • Implementation of Directories File Systems
Fundamental ambiguity • Is the file the “container of the information” or the “information” itself? • Almost all systems confuse the two. • Almost all people confuse the two. File Systems
Example – Suppose that you e-mail me a document • Later, how do either of us know that we are using the same version of the document? • Windows/Outlook/Exchange/MacOS: • Time-stamp is a pretty good indication that they are • Time-stamps preserved on copy, drag and drop, transmission via e-mail, etc. • Unix/Linux • By default, time-stamps not preserved on copy, ftp, e-mail, etc. • Time-stamp associated with container, not with information File Systems
Rule of Thumb • Almost always, application thinks in terms of the information • Most systems think in terms of containers Professional Guidance: Be aware of the distinction, even when the system is not File Systems
Name: Although the name is not always what you think it is! Type: May be encoded in the name (e.g., .cpp, .txt) Dates: Creation, updated, last accessed, etc. (Usually) associated with container Better if associated with content Size: Length in number of bytes; occasionally rounded up Protection: Owner, group, etc. Authority to read, update, extend, etc. Locks: For managing concurrent access … Attributes of Files File Systems
File Metadata • Information about a file • Maintained by the file system • Separate from file itself • Possibly attached to the file • E.g., in block #-1 • Used by OS in variety of ways • Occasionally used by application File Systems
File Types File Systems
Non-attribute of files – Location • Example 1: mv ~lauer/project1.doc ~cs502/public_html/S06 • Example 2: • System moves file from disk block 10,000 to disk block 20,000 • System restores a file from backup • May or may not be reflected in metadata File Systems
Attribute anomaly – Unix File Naming ln ~lauer/project1.doc ~cs502/public_html/S06 ln ~lauer/project1.doc NewProject1.doc • Unix hard links allow one file to have more than one name and/or location • The real name of a Unix file is its i-node – an internal name known only to the OS; points to metadata • Hard links are not used very often in modern Unix practice • Usually safe to regard last element of path as name File Systems
Operations on Files • Open, Close • Gain or relinquish access to a file • OS returns a file handle – an internal data structure letting it cache internal information needed for efficient file access • Read, Write, Truncate • Read: return a sequence of n bytes from file • Write: replace n bytes in file, and/or append to end • Truncate: throw away all but the first n bytes of file • Seek, Tell • Seek: reposition file pointer for subsequent reads and writes • Tell: get current file pointer • Create, Delete: • Conjure up a new file; or blow away an existing one File Systems
File – a very powerful abstraction • Documents, code • Databases • Very large, possibly spanning multiple disks • Streams • Input, output, keyboard, display • Pipes, network connections, … • Virtual memory backing store • … File Systems
File Access Methods • Sequential access • Read all bytes/records from the beginning • Cannot jump around, could possibly rewind or back up • Convenient when medium was magnetic tape or punched cards • Random access • Bytes/records read in any order • Essential for data base systems • Read can be … • move file pointer (seek), then read or … • read and then move file marker • Keyed or indexed access – access items in file based on the contents of an (part of an) item in the file • Provided in older commercial operating systems (IBM ISAM) • (Usually) handled separately by database applications in modern systems File Systems
Questions? File Systems
Directory – A Special Kind of File • A tool for users & applications to organize and find files • User-friendly names • Names that are meaningful over long periods of time • The data structure for OS to locate files (i.e., containers) on disk File Systems
Directory structures • Single level • One directory per system, one entry pointing to each file • Small, single-user or single-use systems • PDA, cell phone, etc. • Two-level • Single “master” directory per system • Each entry points to one single-level directory per user • Uncommon in modern operating systems • Hierarchical • Any directory entry may point to • Individual file • Another directory • Used in most modern operating systems File Systems
Directory Considerations • Efficiency – locating a file quickly. • Naming – convenient to users. • Separate users can use same name for separate files. • The same file can have different names for different users. • Names need only be unique within a directory • Grouping – logical grouping of files by properties • e.g., all Java programs, all games, … File Systems
Directory Organization – Hierarchical • Most systems support idea of current (working) directory • Absolute names – fully qualified from root of file system • /usr/group/foo.c • Relative names – specified with respect to working directory • foo.c • A special name – the working directory itself • “.” • Modified Hierarchical – Acyclic Graph (no loops) and General Graph • Allow directories and files to have multiple names • Links are file names (directory entries) that point to existing (source) files File Systems
Links • Hard links: bi-directional relationship between file names and file • A hard link is directory entry that points to a source file’s metadata • Metadata maintains reference count of the number of hard links pointing to it – link reference count • Link reference count is decremented when a hard link is deleted • File data is deleted and space freed when the link reference count goes to zero • Symbolic (soft) links: uni-directional relationship between a file name and the file • Directory entry points to new metadata • Metadata points to the source file • If the source file is deleted, the link pointer is invalid File Systems
Path Name Translation • Assume that I want to open “/home/lauer/foo.c” fd = open(“/home/lauer/foo.c”, O_RDWR); • File System does the following • Opens directory “/” – the root directory is in a known place on disk • Search root directory for the directory home and get its location • Open home and search for the directory lauer and get its location • Open lauer and search for the file foo.c and get its location • Open the file foo.c • Note that the process needs the appropriate permissions • File Systems spend a lot of time walking down directory paths • This is why open calls are separate from other file operations • File System attempts to cache prefix lookups to speed up common searches File Systems
Directory Operations • Create: • Make a new directory • Add, Delete entry: • Invoked by file create & destroy, directory create & destroy • Find, List: • Search or enumerate directory entries • Rename: • Change name of an entry without changing anything else about it • Link, Unlink: • Add or remove entry pointing to another entry elsewhere • Introduces possibility of loops in directory graph • Destroy: • Removes directory; must be empty File Systems
More on Directories • Fundamental mechanism for interpreting file names in an operating system • Orphan: a file not named in any directory • Cannot be opened by any application (or even OS) • (Often) does not even have name! • Tools • FSCK – check & repair file system, find orphans • Delete_on_close attribute • Special directory entry: “..” parent in hierarchy • Essential for maintaining integrity of directory system • Useful for relative naming File Systems
Break File Systems
Implementation of Files • Map file abstraction to physical disk blocks • Some goals • Efficient in time, space, use of disk resources • Fast enough for application requirements • Effective for a wide variety of file sizes • Many small files (< 1 page) • Huge files (100’s of gigabytes, terabytes, spanning disks) • Everything in between File Systems
File Allocation Schemes • Contiguous • Blocks of file stored in consecutive disk sectors • Directory points to first entry • Linked • Blocks of file scattered across disk, as linked list • Directory points to first entry • Indexed • Separate index block contains pointers to file blocks • Directory points to index block File Systems
Contiguous Allocation • Ideal for large, static files • Databases, fixed system structures, OS code • CD-ROM, DVD • Simple address calculation • Block address sector address • Fast multi-block reads and writes • Minimize seeks between blocks • Prone to fragmentation when … • Files come and go • Files change size • Similar to unpaged virtual memory File Systems
Bad Block Management –Contiguous Allocation • Bad blocks must be concealed • Foul up the block-to-sector calculation • Methods • Spare sectors in each track, remapped by formatting • Look-aside list of bad sectors • Check each sector request against hash table • If present, substitute a replacement sector behind the scene • Handling • Disk controller, invisible to OS • Lower levels of OS file system; invisible to application File Systems
Contiguous Allocation – Extents • Extent: a contiguously allocated subset of a file • Directory entry contains • (For file with one extent) the extent itself • (For file with multiple extents) pointer to an extent block describing multiple extents • Advantages • Speed, ease of address calculation of contiguous file • Avoids (some of) the fragmentation issues • Can be extended to support files across multiple disks • Disadvantages • Degenerates to indexed allocation in Unix-like systems File Systems
Blocks scattered across disk Each block contains pointer to next block Directory points to first and last blocks Sector header: Pointer to next block ID and block number of file Linked Allocation File Systems
Linked Allocation • Advantages • No space fragmentation! • Easy to create, extend files • Ideal for lots of small files • Disadvantages • Lots of disk arm movement • Space taken up by links • Sequential access only! File Systems
Variation on Linked Allocation – File Allocation Table • Instead of a link oneach block, put alllinks in one table • the FAT (File Allocation Table) • One entry per physical block in disk • Directory points to first & last blocks of file • Each block points to next block (or EOF) File Systems
FAT File Systems • Advantages • Advantages of Linked File System • FAT can be cached in memory • Searchable at CPU speeds, pseudo-random access • Disadvantages • Limited size, not suitable for very large disks • FAT cache describes entire disk, not just open files! • Not fast enough for large databases • Used in MS-DOS, early Windows systems File Systems
Disk Defragmentation • Re-organize blocks in disk so that file is (mostly) contiguous • Link or FAT organization preserved • Minimizes disk arm movement during sequential accesses File Systems
Bad Block Management –Linked and FAT Systems • In OS:– format all sectors of disk • Don’t reserve any spare sectors • Allocate bad blocks to a hidden file for the purpose • If a block becomes bad, append to the hidden file • Advantages • Very simple • No look-aside or sector remapping needed • Totally transparent without any hidden mechanism File Systems
Indexed Allocation • i-node: • Part of file metadata • Data structure lists the sector address of each block of file • Advantages • True random access • Only i-nodes of open files need to be cached • Supports small and large files File Systems
Unix i-nodes • Direct blocks: • Pointers to first n sectors • Single indirect table: • Extra block containing pointers to blocks n+1 .. n+m • Double indirect table: • Extra block containing single indirect blocks • … File Systems
Indexed Allocation • Access to every block of file is via i-node • Bad block management • Same as Linked/FAT systems • Disadvantage • Not as fast as contiguous allocation for large databases • Requires reference to i-node for every access vs. • Simple calculation of block to sector address File Systems
Questions? File Systems
Implementation of Directories • A list of [name, information] pairs • Must be scalable from very few entries to very many • Name: • User-friendly, variable length • Any language • Fast access by name • Information: • File metadata (itself) • Pointer to file metadata block (or i-node) on disk • Pointer to first & last blocks of file • Pointer to extent block(s) • … File Systems
name1 attributes name2 attributes name3 attributes name4 attributes … … Very Simple Directory • Short, fixed length names • Attribute & disk addresses contained in directory • MS-DOS, etc. File Systems
name1 name2 name3 name4 … Simple Directory i-node i-node i-node i-node Data structurescontaining attributes • Short, fixed length names • Attributes in separate blocks (e.g., i-nodes) • Attribute pointers are disk addresses (or i-node numbers) • Older Unix versions File Systems
attributes attributes attributes attributes … … name1 longer_name3 very_long_name4 name2 … More Interesting Directory • Variable length file names • Stored in heap at end • Modern Unix, Windows • Linear or logarithmic search for name • Compaction needed after • Deletion, Rename File Systems
Very Large Directories • Hash-table implementation • Each hash chain like a small directory with variable-length names • Must be sorted for listing File Systems
Free Block Management • Bitmap • Very compact on disk • Expensive to search • Supports contiguous allocation • Free list • Linked list of free blocks • Each block contains pointer to next free block • Only head of list needs to be cached in memory • Very fast to search and allocate • Contiguous allocation vary difficult File Systems
Free Block ManagementBit Vector 0 1 2 n-1 … 0 block[i] free 1 block[i] occupied bit[i] = Free block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit File Systems