Files and File Systems CS 502 Operating Systems

Files and File Systems CS 502 – Operating Systems Spring 2006 CS 502 Spring 2006 Files and File Systems 1

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 • (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! CS 502 Spring 2006 Files and File Systems 2

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 CS 502 Spring 2006 Files and File Systems 3

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. CS 502 Spring 2006 Files and File Systems 4

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: • Time-stamp is a pretty good indication that they are • Time-stamps preserved on copy, drag and drop, transmission via e-mail, etc. • Unix • By default, time-stamps not preserved on copy, ftp, e -mail, etc. • Time-stamp associated with container, not with information CS 502 Spring 2006 Files and File Systems 5

Rule of Thumb • Almost always, application (and user) 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 CS 502 Spring 2006 Files and File Systems 6

Attributes of Files • Name: • Size: – Although the name may not be 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 CS 502 Spring 2006 – Length in number of bytes; occasionally rounded up • Protection: – Owner, group, etc. – Authority to read, update, extend, etc. • Locks: – For managing concurrent access • … Files and File Systems 7

File Metadata • Attributes of the file maintained by the file system – Separate from file itself – Possibly attached to the file – Used by OS in variety of ways CS 502 Spring 2006 Files and File Systems 8

File Types CS 502 Spring 2006 Files and File Systems 9

Non-attribute of files – Location • Example 1: mv ~lauer/project 1. doc ~cs 502/public_html/S 06 • Example 2: – System moves file from disk block 10, 000 to disk block 20, 000 – System restores a file from backup CS 502 Spring 2006 Files and File Systems 10

Attribute anomaly – Unix File Naming ln ~lauer/project 1. doc ~cs 502/public_html/S 06 ln ~lauer/project 1. doc New. Project 1. 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 • Hard links are rarely used in modern Unix practice – Usually safe to regard last element of path as name CS 502 Spring 2006 Files and File Systems 11

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 CS 502 Spring 2006 Files and File Systems 12

File – a very powerful abstraction • Documents, code • Databases • Very large, possibly spanning multiple disks • Streams • Input, output, keyboard, display • Pipes • Virtual memory backing store • … CS 502 Spring 2006 Files and File Systems 13

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 • 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 CS 502 Spring 2006 Files and File Systems 14

File Access Methods (continued) • 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) – (Today) usually handled separately by database applications in modern systems CS 502 Spring 2006 Files and File Systems 15

Directory – A Special Kind of File • A tool for users & applications to organize and 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 CS 502 Spring 2006 Files and File Systems 16

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 • Another directory • Individual file – Used in most modern operating systems CS 502 Spring 2006 Files and File Systems 17

Directory Organization – Hierarchical CS 502 Spring 2006 Files and File Systems 18

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, … CS 502 Spring 2006 Files and File Systems 19

More on Hierarchical Directories • 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 CS 502 Spring 2006 Files and File Systems 20

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 counts the number of hard links (including the source file) – 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 is 0 • 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 CS 502 Spring 2006 Files and File Systems 21

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 • Some 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 CS 502 Spring 2006 Files and File Systems 22

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 CS 502 Spring 2006 Files and File Systems 23

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 – when number of links reaches zero • Special directory entry: “. . ” parent in hierarchy • Essential for maintaining integrity of directory system • Useful for relative naming CS 502 Spring 2006 Files and File Systems 24

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 CS 502 Spring 2006 Files and File Systems 25

File Allocation Schemes • Contiguous – Blocks of file stored in consecutive disk sectors – Directory points to first entry, others derived by math • Linked – Blocks of file scatter across disk, as linked list – Directory points to first entry, follow link to find others • Indexed – Separate index block contains pointers to file blocks – Directory points to index block, lookup all blocks CS 502 Spring 2006 Files and File Systems 26

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 CS 502 Spring 2006 Files and File Systems 27

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 appl. CS 502 Spring 2006 Files and File Systems 28

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 CS 502 Spring 2006 Files and File Systems 29

Linked Allocation • Blocks scattered across disk – Each block contains pointer to next block – Directory points to first and last blocks – Sector header block: • Pointer to next block • ID and block number of file CS 502 Spring 2006 Files and File Systems 30

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! CS 502 Spring 2006 Files and File Systems 31

Variation on Linked Allocation – File Allocation Table • Instead of a link on each block, put all links in one table – the FAT (File Allocation Table) – fixed place on disk • One entry per physical block in disk – Directory points to first & last blocks of file – Each block points to next block (or EOF) CS 502 Spring 2006 Files and File Systems 32

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 CS 502 Spring 2006 Files and File Systems 33

Disk Defragmentation • Re-organize blocks in disk so that file is (mostly) contiguous • Linked or FAT organization preserved • Minimizes disk arm movement during sequential accesses CS 502 Spring 2006 Files and File Systems 34

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 CS 502 Spring 2006 Files and File Systems 35

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 CS 502 Spring 2006 Files and File Systems 36

Unix i-nodes – Part of File Metadata • 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 • … CS 502 Spring 2006 Files and File Systems 37

Indexed Allocation • Access to every block of file is via i-node • Bad block management – Same as Linked/FAT systems • Disadvantage – Not as fast a contiguous allocation for large databases CS 502 Spring 2006 Files and File Systems 38

Next time • Implementing Directories • Other “disk” systems – CD-ROM – RAID (Redundant Array of Inexpensive Disks) • Stable Storage • Log File Systems CS 502 Spring 2006 Files and File Systems 39