File Systems: Abstraction, Organization, and Access Methods, Slides of Advanced Operating Systems

Download File Systems: Abstraction, Organization, and Access Methods and more Slides Advanced Operating Systems in PDF only on Docsity! Lecture No. 29 docsity.com File systems  The implementation of the abstraction for secondary storage  Logical organization of files into directories  Sharing of data between processes, people and machines docsity.com File access methods  Sequential access  read bytes one at a time, in order  Direct access  random access given a block/byte number  Record access  file is array of fixed- or variable-sized records  Indexed access  FS contains an index to a particular field of each record in a file  apps can find a file based on value in that record (similar to DB) docsity.com Directories  Most file systems support multi-level directories  naming hierarchies (/, /usr, /usr/local, /usr/local/bin, …)  Most file systems support the notion of current directory  absolute names: fully-qualified starting from root of FS bash$ cd /usr/local  relative names: specified with respect to current directory bash$ cd /usr/local (absolute) bash$ cd bin (relative, equivalent to cd /usr/local/bin) docsity.com Path name translation  Let’s say you want to open “/one/two/three” fd = open(“/one/two/three”, O_RDWR); docsity.com rT Disk Layout Strategies 1. Contiguous allocation » Like memory » Fast, simplifies directory access » Inflexible, causes fragmentation, needs compaction 2. Linked structure 3. Indexed structure (indirection, hierarchy) docsity.com Simple mechanism: contiguous allocation file a (base=1,len=3) file b (base=5,len=2) what happens if file c needs 2 sectors??? docsity.com Linked files  pro: easy dynamic growth & sequential access, no fragmentation  con?  Examples (sort-of): Alto, TOPS-10, DOS FAT file a (base=1) file b (base=5) how do you find the last block in a? docsity.com