================ 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
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
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.
5.4.2: Disk Formatting
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
FCFS (First Come First Served): Simple but has long delays.
Pick: Same as FCFS but pick up requests for cylinders that are
passed on the way to the next FCFS request.
SSTF or SSF (Shortest Seek (Time) First): Greedy algorithm. Can
starve requests for outer cylinders and almost always favors middle
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
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.
N-step Scan: This is what the natural implementation of Scan
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
Compare this to selfish round robin (SRR)
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.
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
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.
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
This is presumably what you did for the (processor) scheduling
Accounting: At each tick, bump a counter in the process table
entry for the currently running process.
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
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.
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.
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
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
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.
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.
down-cntl down-x up-x up-cntl is converted to cntl-x
down-cntl up-cntl down-x up-x is converted to x
down-cntl down-x up-cntl up-x is converted to cntl-x (I just
tried it to be sure).
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
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
5.7: Graphical User Interfaces (GUIs)
5.8: Network Terminals
5.9: Power Management
5.10: Research on Input/Output
Chapter 6: File Systems
Size: Store very large amounts of data.
Persistence: Data survives the creating process.
Access: Multiple processes can access the data concurrently.
Solution: Store data in files that together form a file system.
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
Extensions, e.g. the “html” in
Depending on the system, these can have little or great
The extensions can be
Conventions just for humans: letter.teq (my convention).
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
Default behavior for Operating system or window manager or
Click on .xls file in windows and excel is started.
Click on .xls file in nautilus under linux and open office
Required extensions for programs
The gnu C compiler (and probably others) requires C
programs be named *.c and assembler programs be named *.s
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.
Unix: yes. Windows: no.
6.1.2: File structure
A file is a
Unix, dos, windows.
- (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
Varied and complicated beast.
Supports rapidly finding a record with a specific
Supports retrieving (varying size) records in key order.
Treated in depth in database courses.
6.1.3: File types
- (Regular) files.
Directories: studied below.
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
“Symbolic” Links (similar to “shortcuts”): Also studied
“Magic number”: Identifies an executable file.
There can be several different magic numbers for different types
- unix: #!/usr/bin/perl
Strongly typed files:
The type of the file determines what you can do with the
This make the easy and (hopefully) common case easier and, more
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.