Outline for 2/18/98

• Last lecture: finished memory management
• Administrative
• The answer is "not yet"
• Homework 4 on web.
• Program 3 coming soon.
• Objective: overview of file systems issues that we will be covering in next few lectures.

• What is the role of files?
  What is the file abstraction?
• File naming. How to find the file we want?
  Sharing files. Controlling access to files.
• Performance issues - how to deal with the bottleneck of disks?
  What is the "right" way to optimize file access?

• Persistance - long-lived - data for posterity
• non-volitile storage media
• semantically meaningful (memorable) names

*File Abstractions

• UNIX-like files
• Sequence of bytes
• Operations: open (create), close, read, write, seek
• Memory mapped files
• Sequence of bytes
• Mapped into address space
• Page fault mechanism does data transfer
• Named, Possibly typed

• (Directory subsystem) Map filenames to fileids-open (create) syscall. Create kernel data structures.
  Maintain naming structure (unlink, mkdir, rmdir)
• Determine layout of files and metadata on disk in terms of blocks. Disk block allocation. Bad blocks.
• Handle read and write system calls
• Initiate I/O operations for movement of blocks to/from disk.
• Maintain buffer cache

In general, deal with device characteristics

• Translate block numbers (the abstraction of device shown to file system) to physical disk addresses.
  Device specific (subject to change with upgrades in technology) intelligent placement of blocks.
• Schedule (reorder?) disk operations

• File usage patterns should influence design decisions. Do things differently depending:
• How large are most files? How long-lived?
  Read vs. write activity. Shared often?
• Different levels "see" a different workload.
• Feedback loop