# Operating Systems

================ Start Lecture #25 ================

#### RAID (Redundant Array of Inexpensive Disks)

• 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+1=N (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.

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### 5.4.3: Disk Arm Scheduling Algorithms

There are three components to disk response time: seek, rotational latency, and transfer time. Disk arm scheduling is concerned with minimizing seek time by reordering the requests.

These algorithms are relevant only if there are several I/O requests pending. For many PCs this is not the case. For most commercial applications, I/O is crucial and there are often many requests pending.

1. FCFS (First Come First Served): Simple but has long delays.

2. Pick: Same as FCFS but pick up requests for cylinders that are passed on the way to the next FCFS request.

3. SSTF or SSF (Shortest Seek (Time) First): Greedy algorithm. Can starve requests for outer cylinders and almost always favors middle requests.

4. Scan (Look, Elevator): The method used by an old fashioned jukebox (remember “Happy Days”) and by elevators. The disk arm proceeds in one direction picking up all requests until there are no more requests in this direction at which point it goes back the other direction. This favors requests in the middle, but can't starve any requests.

5. C-Scan (C-look, Circular Scan/Look): Similar to Scan but only service requests when moving in one direction. When going in the other direction, go directly to the furthest away request. This doesn't favor any spot on the disk. Indeed, it treats the cylinders as though they were a clock, i.e. after the highest numbered cylinder comes cylinder 0.

6. N-step Scan: This is what the natural implementation of Scan gives.
• While the disk is servicing a Scan direction, the controller gathers up new requests and sorts them.
• At the end of the current sweep, the new list becomes the next sweep.
• Compare this to selfish round robin (SRR) with b≥a=0.

#### Minimizing Rotational Latency

Use Scan based on sector numbers not cylinder number. For rotational latency Scan is the same as C-Scan. Why?
Ans: Because the disk only rotates in one direction.

Homework: 24, 25

### 5.4.4: Error Handling

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

## 5.5: Clocks

Also called timers.

### 5.5.1: 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.

### 5.5.2: Clock Software

1. Time of day (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 (i.e., changes the state of the process from ready to running). This is presumably what you did for the (processor) scheduling lab.

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. There is one node for each event.
• The first entry in each node is the time after the preceding event that this event's alarm is to ring.
• For example, if the time is zero, this event occurs at the same time as the previous event.
• The second entry in the node 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 following 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: 27

## 5.6: Character-Oriented Terminals

### 5.6.1: RS-232 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.

#### 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.

### 5.6.2: Input Software

• We are just looking at keyboard input. Once again graphics is too involved to be treated well.
• 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).

### 5.6.3: Output Software

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

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# Chapter 6: File Systems

Requirements

1. Size: Store very large amounts of data.
2. Persistence: Data survives the creating process.
3. Access: Multiple processes can access the data concurrently.

Solution: Store data in files that together form a file system.

## 6.1: Files

### 6.1.1: File Naming

Very important. A major function of the file system.

• Does each file have a unique name?
Answer: Often no. We will discuss this below when we study links.

• Extensions, e.g. the “html” in “class-notes.html”. Depending on the system, these can have little or great significance. The extensions can be

1. Conventions just for humans: letter.teq (my convention).

2. Conventions giving default behavior for some programs.
• The emacs editor thinks .html files should be edited in html mode but
can edit them in any mode and can edit any file in html mode.
• Firefox thinks .html means an html file, but
<html> ... </html> works as well
• Gzip thinks .gz means a compressed file but accepts a --suffix flag

3. Default behavior for Operating system or window manager or desktop environment.
• Click on .xls file in windows and excel is started.
• Click on .xls file in nautilus under linux and open office is started.

4. Required extensions for programs
• The gnu C compiler (and probably others) requires C programs be named *.c and assembler programs be named *.s

5. Required extensions by operating systems
• MS-DOS treats .com files specially
• Windows 95 requires (as far as I could tell) shortcuts to end in .lnk.

• Case sensitive?
Unix: yes. Windows: no.

### 6.1.2: File structure

A file is a

1. Byte stream
• Unix, dos, windows.
• Maximum flexibility.
• Minimum structure.

2. (fixed size) Record stream: Out of date
• 80-character records for card images.
• 133-character records for line printer files. Column 1 was for control (e.g., new page) Remaining 132 characters were printed.

3. Varied and complicated beast.
• Indexed sequential.
• B-trees.
• Supports rapidly finding a record with a specific key.
• Supports retrieving (varying size) records in key order.
• Treated in depth in database courses.

### 6.1.3: File types

Examples

1. (Regular) files.

2. Directories: studied below.

3. Special files (for devices). Uses the naming power of files to unify many actions.
dir             # prints on screen
dir > file      # result put in a file
dir > /dev/tape # results written to tape

4. “Symbolic” Links (similar to “shortcuts”): Also studied below.

“Magic number”: Identifies an executable file.

• There can be several different magic numbers for different types of executables.
• unix: #!/usr/bin/perl

Strongly typed files:

• The type of the file determines what you can do with the file.
• This make the easy and (hopefully) common case easier and, more importantly, safer.
• It tends to make the unusual case harder. For example, you have a program that turns out data (.dat) files. But you want to use it to turn out a java file but the type of the output is data and cannot be easily converted to type java.