RAID (Redundant Array of Inexpensive Disks)

• Tanenbaum's treatment is not very good.
  
  
• The name RAID is from Berkeley.
  
  
• IBM changed the name to Redundant Array of Independent Disks. I wonder why?
  
  
• A simple form is mirroring, where two disks contain the same data.
  
  
• Another simple form is striping (interleaving) where consecutive blocks are spread across multiple disks. This helps bandwidth, but is not redundant. Thus it shouldn't be called RAID, but it sometimes is.
  
  
• One of the normal RAID methods is to have N (say 4) data disks and one parity disk. Data is striped across the data disks and the bitwise parity of these sectors is written in the corresponding sector of the parity disk.
  
  
• On a read if the block is bad (e.g., if the entire disk is bad or even missing), the system automatically reads the other blocks in the stripe and the parity block in the stripe. Then the missing block is just the bitwise exclusive or of all these blocks.
  
  
• For reads this is very good. The failure free case has no penalty (beyond the space overhead of the parity disk). The error case requires N+1 (say 5) reads.
  
  
• A serious concern is the small write problem. Writing a sector requires 4 I/O. Read the old data sector, compute the change, read the parity, compute the new parity, write the new parity and the new data sector. Hence one sector I/O became 4, which is a 300% penalty.
  
  
• Writing a full stripe is not bad. Compute the parity of the N (say 4) data sectors to be written and then write the data sectors and the parity sector. Thus 4 sector I/Os become 5, which is only a 25% penalty and is smaller for larger N, i.e., larger stripes.
  
  
• A variation is to rotate the parity. That is, for some stripes disk 1 has the parity, for others disk 2, etc. The purpose is to not have a single parity disk since that disk is needed for all small writes and could become a point of contention.
  
  

5.3.3: Error Handling

Disks error rates have dropped in recent years. Moreover, bad block forwarding is done by the controller (or disk electronic) so this topic is no longer as important for OS.

5.3.4: Track Caching

Often the disk/controller caches a track, since the seek penalty has already been paid. In fact modern disks have megabyte caches that hold recently read blocks. Since modern disks cheat and don't have the same number of blocks on each track, it is better for the disk electronics (and not the OS or controller) to do the caching since it is the only part of the system to know the true geometry.

5.3.5: Ram Disks

• Fairly clear. Organize a region of memory as a set of blocks and pretend it is a disk.
• A problem is that memory is volatile.
• Often used during OS installation, before disk drivers are available (there are many types of disk but all memory looks the same so only one ram disk driver is needed).

5.4: Clocks

Also called timers.

5.4.1: Clock Hardware

• Generates an interrupt when timer goes to zero
• Counter reload can be automatic or under software (OS) control.
• If done automatically, the interrupt occurs periodically and thus is perfect for generating a clock interrupt at a fixed period.

5.4.2: Clock Software

1. TOD: Bump a counter each tick (clock interupt). If counter is only 32 bits must worry about overflow so keep two counters: low order and high order.
   
   
2. Time quantum for RR: Decrement a counter at each tick. The quantum expires when counter is zero. Load this counter when the scheduler runs a process.
   
   
3. Accounting: At each tick, bump a counter in the process table entry for the currently running process.
   
   
4. Alarm system call and system alarms:
   • Users can request an alarm at some future time.
   • The system also on occasion needs to schedule some of its own activities to occur at specific times in the future (e.g. turn off the floppy motor).
   • The conceptually simplest solution is to have one timer for each event.
   • Instead, we simulate many timers with just one.
   • The data structure on the right works well.
   • The time in each list entry is the time after the preceding entry that this entry's alarm is to ring.
   • For example, if the time is zero, this event occurs at the same time as the previous event.
   • The other entry is a pointer to the action to perform.
   • At each tick, decrement next-signal.
   • When next-signal goes to zero, process the first entry on the list and any others following immediately after with a time of zero (which means they are to be simultaneous with this alarm). Then set next-signal to the value in the next alarm.
5. Profiling
   • Want a histogram giving how much time was spent in each 1KB (say) block of code.
   • At each tick check the PC and bump the appropriate counter.
   • A user-mode program can determine the software module associated with each 1K block.
   • If we use finer granularity (say 10B instead of 1KB), we get increased accuracy but more memory overhead.

Homework: 12

5.5: Terminals

5.5.1: Terminal Hardware

Quite dated. It is true that modern systems can communicate to a hardwired ascii terminal, but most don't. Serial ports are used, but they are normally connected to modems and then some protocol (SLIP, PPP) is used not just a stream of ascii characters. So skip this section.

5.5.2: Memory-Mapped Terminals

Not as dated as the previous section but it still discusses the character not graphics interface.

• Today, software writes into video memory the bits that are to be put on the screen and then the graphics controller converts these bits to analog signals for the monitor (actually laptop displays and some modern monitors are digital).
• But it is much more complicated than this. The graphics controllers can do a great deal of video themselves (like filling).
• This is a subject that would take many lectures to do well.
• I believe some of this is covered in 201.

Keyboards

Tanenbaum description of keyboards is correct.

• At each key press and key release a code is written into the keyboard controller and the computer is interrupted.
• By remembering which keys have been depressed and not released the software can determine Cntl-A, Shift-B, etc.

5.5.3: Input Software

• We are just looking at keyboard input. Once again graphics is too involved to be treated here.
• There are two fundamental modes of input, sometimes called raw and cooked.
• In raw mode the application sees every ``character'' the user types. Indeed, raw mode is character oriented.
  • All the OS does is convert the keyboard ``scan codes'' to ``characters'' and and pass these characters to the application.
  • Some examples
    1. down-cntl down-x up-x up-cntl is converted to cntl-x
    2. down-cntl up-cntl down-x up-x is converted to x
    3. down-cntl down-x up-cntl up-x is converted to cntl-x (I just tried it to be sure).
    4. down-x down-cntl up-x up-cntl is converted to x
  • Full screen editors use this mode.
• Cooked mode is line oriented. The OS delivers lines to the application program.
  • Special characters are interpreted as editing characters (erase-previous-character, erase-previous-word, kill-line, etc).
  • Erased characters are not seen by the application but are erased by the keyboard driver.
  • Need an escape character so that the editing characters can be passed to the application if desired.
  • The cooked characters must be echoed (what should one do if the application is also generating output at this time?)
• The (possibly cooked) characters must be buffered until the application issues a read (and an end-of-line EOL has been received for cooked mode).

5.5.4: Output Software

Again too dated and the truth is too complicated to deal with in a few minutes.

Homework: 16.