Files and Disks

• the File Manager manages files, directories (collections of files), and file systems (collections of directories)
• a file is a named collection of data stored on a device
• the file manager is responsible for maintaining the file abstraction as a way of hiding details of long-term storage
• in other words, disk drives don't really know what a file is
• consider what happens when a disk begins to get full and there's not enough contiguous space for a large file
• a suitcase won't fit in a trunk? let the file manager take care of it by sawing the suitcase into pieces and fit them each in the smaller, available spaces
• a disk read or write typically works with a group of bytes called a block (sometimes called a cluster)
• a file is really just a name associated with a collection of blocks on the device
• this implies there is a table of contents somewhere that allows the file manager to find the blocks given a file name
• a disk can be formatted to use relatively small blocks (e.g. 512KB) or large blocks (e.g. 8MB)
• in general devices can be categorized as block devices (e.g. hard drives, DVD drives) or character devices (e.g. keyboards,
• a disk is organized in tracks and sectors
• each disk drive typically has multiple platters and the read/write head moves across each
• the same track on all platters makes a cylinder
• consider the physical movements involved in retrieving data from a hard disk
• most drivers optimize the requests by attempting to minimize movements
• contrast these strategies (see handout for details): FCFS, SSTF, Scan/Look, Circular Scan/Look
• consider how blocks are allocated when saving a file
• some possible strategies: contiguous, linked list, indexed
• most systems use indexed, which operates much like the page directories of a memory manager
• fixed-sized blocks leads to internal fragmentation
• a defragger is designed to recover some of what is lost via internal fragmentation (also rearranges blocks to be contiguous)
• still, it calls for a wise choice in block size
• since a directory really just has a pointer to a file, a file can show up in multiple directories (Unix does this with links)
• ultimately all directories have to report to somebody; even the top-level directories have to adhere to some naming scheme
• this global naming scheme is really the file system; file systems fit into the operating system's scheme
• consider how mounting a file system works

Streams and Buffers

• consider how the file manager manages files
• the device doesn't just store the file's contents; it also keeps various attributes (e.g. owner, permissions)
• every time a file is opened, the file manager creates a file descriptor to manage the interaction between application and file
• the application views a file as a stream of bytes with a file pointer; however this is not how the data is actually stored
• data goes from sectors and tracks interpreted as blocks which are loaded into buffers interpreted as streams
• consider the effect of a buffer when reading and writing (note particularly what happens when we insert text into a file)
• in addition to streams, some programming environments allow the data to be interpreted as records, or its modern equivalent: objects
• a persistent object is backed by a file or database
• historically this was referred to as record-level processing, a typical pattern for COBOL programs and early databases
• in the end, each layer deals with different abstractions at the appropriate level of detail

RAID

• RAID is Redundant Array of Inexpensive Disks (note: some say the I stands for Independent)
• RAID creates a layer of abstraction between the file manager and the device
• in particular, RAID software translates between logical blocks and physical blocks
• the file manager translates physical blocks on a device into logical buffers in RAM
• RAID does not change this, but it provides a layer of virtual devices above the physical devices
• this layer of abstraction allows the contents of a file to be distributed and/or duplicated across multiple hard drives
• possible benefits of this include better performance, better reliability and recoverability
• RAID comes in different configurations or levels
• consider how splitting a file across multiple disks can improve performance
• consider how duplicating a file across multiple disks can improve reliability/recoverability