quartz/content/notes/15-file-systems.md

title	aliases	tags	sr-due	sr-interval	sr-ease
15-file-systems		cosc204 lecture	2022-09-25	3	250

what is a file (LINUX)

an interface of OS to users

os manages files just as files - it doesn't care what type of file it is - they are all bytes arranges logically in sequence.

they are arrangges logically in sequence

files need to be interpreted. THis is done by applications. e.g., powerpoint, ms paint, etc.

• the OS provides a logical unit of storage for the user
  • the user refers to files
  • the operating system maps files onto regions of the storage
  • files are really an artifact of the dialogue between the user and the OS

how to define it

• a collection of related data
• e.g., the set of lines in a program

structure:

• a byte stream or a sequence of bytes, text file is a stream of ascii characgers

what can the user do

• create, write, read, reposition, delete, truncate (through system calls)

attributes

• name
• owner
  • Uid and Gid
• file type
• location of data
• size of data
• permission
• "housekeeping" information
  • time and created modified etc
• and more

where is the data stored

• in an index node (inode) pointed by an entry of a directory. (you can think of a directory as just a set of <name, inode> entries. )

internal fragmentation

• dont know exactly how many bytes to allocate to a file
• extra space is lost

external fragmentaion TODO

these issues have been solved using block allocaiton instead of continuous allocation

system calls for file systems

• open close, read, write, ioctl, etc

ssize_t write(int fd, const, void *buf, size_t count) - fd - file descriptor - write up to count butes from the buffer pointed buf to the file referred to by the file descriptor - returns -1 if error. and errno is set - should return number of bytes written if ok

ssize_t read(int fd, void *buf, size_t count)

• attempt to read up to count bytes from the fd into the buffer starting at buf. On success, the numbber of bytes read is returned and the file position is advanced by this number. It may be < count. zero means end of file
• error is -1 and errno is set

file access patterns

• sequential access (most common)
  • a file pointer specifies a record within the file.
  • the poniter can be moved incrementally forwards (in read or write) operations or backwards (in rewinding)
  • the hardware metaphor is a tape
• direct access
  • a file is viewed as a numbered sequence of records
  • operations (e.g., read write) can be carried out on any record in any order
  • the hardware metaphor is a disk

you can give advice to the OS about how to read the file by specifying sequential or direct

directory structure

tree structured

mapped onto a storage device

this is done by the organisation module

smallest unit in a disk is a block

the file organiser has to allocate blocks for the storage of files

a file is broken into logical blocks, to make the mapping to disk blocks easier to manage

file system implementation

disk allocation methods

continious allocation

• each file occupies a set of continuous blocks on the disk
• advantages
  • sequential access is easy becaues the next character is in the current block or the very next block
  • direct access is also good, just count the numbe of blocks
• disadvantages
  • external fragmentation
    • free space is fragmented. the total space is enough for a file but none of the continuous space is large enogh
    • space is not large enough to be allocated
  • internal fragmentation
    • files can grow. and we dont know how big a file is going to be. even if we did, files can shrink too, in which case space is unused
    • this space is allocated but not used

linked allocation

• each file is a linked list of blocks
• the directory contains a pointer to the firsrt (and last) blocks of the file
• each block contains a poniter to the next block
• advantages
  • no external fragmentation
  • files can grow and shrink easily
• disadvantages
  • can only be used effectively for qequential access
  • pointer take up space
  • internal fragmentation (get worse as clusters grow)
  • reliability -> if one pointer is lost, the whole file therafter is also lost

FAT file allocation table

• variation of linked allocation scheme (used in ms-dos)
• a table is created at tthe beginning of each partition
• can also be a point of failure. two copies are kept to safeguard from corruption

indexed allocation