Filesystem

The part of OS that helps programs create, manage, and delete files on disk (sec. storage)

• Logical Level: exposes file and dir. abstractions, offers API calls for file handling
• Physical Level: works w// disk dirmware and moves bytes to/from disk to DRAM

Dozens exist

• differ on how they layer file/dir abstractions as bytes
• differ on how data integrity/reliability is assured
• some can work w/ multiple OS

Virtualization of File on a Disk

OS abstracts a file on disk as a virtual object for processes

• File Descriptor: An OS-assigned integer identifier/reference for a file’s virtual object that a process can use 0/1/2 reserved for STDIN/STDOUT/STDERR
• File Handle: A PL’s abstraction on top of a file descr. (fd)

System Call API for File Handling:

open(): Create a file; assign fd; optionally overwrite read(): Copy file’s bytes on disk to in-mem. buffer; sized write(): Copy bytes from in-mem. buffer to file on disk fsync(): “Flush” (force write) “dirty” data to disk close(): Free up the fd and other OS state info on it lseek(): Position offset in file’s fd (for random R/W later) Dozens more (rename, mkdir, chmod, etc.)

Files Vs Databases: Data Model

• Database: An organized collection of interrelated data
• Data Model: An abstract model to define organization of data in a formal (mathematically precise) way
  • E.g., Relations, XML, Matrices, DataFrames