File SystemsFILE SYSTEMSFile System ComponentsDescriptors for FilesDirectory Structure (structure of relating files to each other)C. Hierarchical (Tree-Structured)D. Aliases: the file appears under several namesOther issuesAccess RightsAccess listsAllocation MethodsC. Methods of allocating spaceComparisonsThe UNIX FILE SYSTEM (From “Unix Implementation”)Unix File System (2)Unix File System (3)File System Partitions10/26/00 CSE 380 1University of PennsylvaniaFile SystemsCSE 380Lecture Note 11bInsup Lee10/26/00 CSE 380 2University of PennsylvaniaFILE SYSTEMS Computer applications need to store and retrieve information:•need to store very large amount of information•need to store information permanently•need to share information A file is a collection of data records grouped together for purpose of access control and modification; a data record is just a linear list of information items. A file system is that software responsible for creating, destroying, organizing, reading, writing, modifying, moving, and controlling access to files; and for management of the resources used by file.10/26/00 CSE 380 3University of PennsylvaniaFile System Components1File Accessing LanguageInterface to user: create, delete, read, write, modify, control the access to files.2Accessing ProceduresRoutines for file directory management and searching, opening and closing files, mapping symbolic file names to their real addresses, controlling the access of files to legitimate users, managing interval buffers and IO programs. 3IO SystemResponsible for maintaining queues of IO requests, scheduling and initiating the operation, servicing IO errors, and handling IO completion signals.4Auxiliary Storage ManagementKeeping track of available space on secondary storage, allocating and deallocating blocks of secondary storage on request. 5Backup and RecoveryEnsures that the file system can be recovered from hardware and software errors.10/26/00 CSE 380 4University of PennsylvaniaDescriptors for Files1File Identification: A symbolic name 2Physical Address. Location and extent of a file3Access Control information (Protection)Who can access and how the file can be used (R/W/E) protection from several simultaneous writes4Historical and Measurement Information (Accounting information)Creation date, date of last change or last read, # of times the file has been opened, and other usage data.5Disposition: Temporary/permanent6Coding of Information Binary(executable), Characters(EBCDIC & ASCII)7Physical or file typeSequential, linked, indexed, fixed/variable records8Logical representation10/26/00 CSE 380 5University of PennsylvaniaDirectory Structure (structure of relating files to each other) A. Single level1each file has a unique name (e.g., Univac Exec-8)2since directory is large, it maybe expensive to search it. B. Two-level1Each file has a two-part name of the form (user, local name)2The “user” identifies a human or a human/project combination3The “name” is meaningful only for that “user”4e.g., Tops-1010/26/00 CSE 380 6University of PennsylvaniaC. Hierarchical (Tree-Structured)1A file has a name with arbitrary member of parts (e.g. UNIX, MULTICES, VMS (8))2Each part needs to be unique only w.r.t. the previous parts3A directory is in many ways a file like any other. (Its special feature is that it is internal to the tree and contains information about the files at the next level under it.)4Extends nicely to a distributed file system10/26/00 CSE 380 7University of PennsylvaniaD. Aliases: the file appears under several names1The file's contents are stored only once.2Other attributes may be stored for each name separately, leading to different attributes for the different names. In UNIX, all attributes except the name are stored exactly once.3Issues: - Who pays for a file that appears in several directories? (Best answers, charge 1/n, where n = # of names that the file has) - What happens when the file is deleted by only one name? (Do not physically remove until it has been deleted under all names) - What about circular paths in the file name space? Allow such paths, and make file search programs smart enough to detect cycles (Multics) or disallow such paths, not allowing directories to have aliases (UNIX)10/26/00 CSE 380 8University of PennsylvaniaOther issues E. Name Convention UNIX VMS Meaningp pas pascal sourceo obj object module file - output from an assembler or compilerout exe linked executable fileF. Version number: a file name includes a version number (e.g., VMS) writing over a file only creates a new version; it does not destroying the previous version G. On Exec-8 (Univac 1100 series) up to 3 files may be indicated by a single name: source, relocatable and executable. Which one to use is determined by its application.10/26/00 CSE 380 9University of PennsylvaniaAccess Rights Possible Rights1Read2Write3Append at the end of the file4Execute5Delete (In Unix, this right is the “write” permission)6Change the rights on the file (In Unix, only the owner has this right; it may not be turned off)10/26/00 CSE 380 10University of PennsylvaniaAccess lists Access rights are stored for each file in an access list.1One for each user in the system2Access rights are specified for each of the following three pools:(a) the owner of the file(b) other people in the owner's group(c) the worldCan't specify. allow A to share files with B allow B to share files with C but prevent A & C from sharing files3Each user owns a list of capabilities which contain files & access rights the given user has.10/26/00 CSE 380 11University of PennsylvaniaAllocation Methods A. Goals1fast sequential access2fast random access3ability to grow4easy allocation of backing store5minimum fragmentation B. Free Space Management1Linked list of free blocks (or tracks or cylinders)Each entry may point to many free blocks, not just one, for efficiency. This method leads to random allocation on the disk. So file access may require a lot of seek activities.2Bit mapEach block (or tracks or cylinders) represented by one bit. Allow clustered allocation10/26/00 CSE 380 12University of PennsylvaniaC. Methods of allocating space1Fixed contiguous regions: Each file occupies a fixed region.Fast seq. & random access, not easy to grow beyond its original intended size; use bit map for allocation2Contiguous regions with overflow areas: Same as 1), but a file that