Start Lecture #14
The ideal storage device is
When compared to central memory, disks are big and cheap, but slow.
Show a real disk opened up and illustrate the components.
Consider the following characteristics of a disk.
Remark: A practice final is available off the web page. Note the format and good luck.
Overlapping I/O operations is important when the system has more than one disk. Many disk controllers can do overlapped seeks, i.e. issue a seek to one disk while another disk is already seeking.
As technology improves the space taken to store a bit decreases, i.e., the bit density increases. This changes the number of cylinders per inch of radius (the cylinders are closer together) and the number of bits per inch along a given track.
Despite what Tanenbaum says later, it is not true that when one head is reading from cylinder C, all the heads can read from cylinder C with no penalty. It is, however, true that the penalty is very small.
Current commodity disks for desktop computers (not for commodity laptops) require about 10ms. before transferring the first byte and then transfer about 40K bytes per ms. (if contiguous). Specifically
This is quite extraordinary. For a large sequential transfer, in the first 10ms, no bytes are transmitted; in the next 10ms, 400,000 bytes are transmitted. The analysis suggests using large disk blocks, 100KB or more.
But the internal fragmentation would be severe since many files are small. Moreover, transferring small files would take longer with a 100KB block size.
In practice typical block sizes are 4KB-8KB.
Multiple block sizes have been tried (e.g. blocks are 8KB but a
file can also have
fragments that are a fraction of
a block, say 1KB).
Some systems employ techniques to encourage consecutive blocks of a given file to be stored near each other. In the best case, logically sequential blocks are also physically sequential and then the performance advantage of large block sizes is obtained without the disadvantages mentioned.
In a similar vein, some systems try to cluster
(e.g., files in the same directory).
Homework: Consider a disk with an average seek time of 5ms, an average rotational latency of 5ms, and a transfer rate of 40MB/sec.
Originally, a disk was implemented as a three dimensional array
Cylinder#, Head#, Sector#
The cylinder number determined the cylinder, the head number specified the surface (recall that there is one head per surface), i.e., the head number determined the track within the cylinder, and the sector number determined the sector within the track.
But there is something wrong here. An outer track is longer (in centimeters) than an inner track, but each stores the same number of sectors. Essentially some space on the outer tracks was wasted.
Later disks lied. They said they had a virtual geometry as above, but really had more sectors on outer tracks (like a ragged array). The electronics on the disk converted between the published virtual geometry and the real geometry.
Modern disk continue to lie for backwards compatibility, but also support Logical Block Addressing in which the sectors are treated as a simple one dimensional array with no notion of cylinders and heads.
The name and its acronym RAID came from Dave Patterson's group at Berkeley. IBM changed the name to Redundant Array of Independent Disks. I wonder why?
The basic idea is to utilize multiple drives to simulate a single larger drive, but with added redundancy.
The different RAID configurations are often called different
levels, but this is not a good name since there is no hierarchy and
it is not clear that higher levels are
better than low ones.
However, the terminology is commonly used so I will follow the trend
and describe them level by level, but having very little to say
about some levels.
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, the system is so underutilized that there are rarely multiple outstanding I/O requests and hence no scheduling is possible. At the other extreme, many large servers, are I/O bound with significant queues of pending I/O requests. For these systems, effective disk arm scheduling is crucial.
Although disk scheduling algorithms are performed by the OS, they are also sometimes implemented in the electronics on the disk itself. The disks I brought to class were somewhat old so I suspect those didn't implement scheduling, but the then-current operating systems definitely did.
We will study the following algorithms all of which are quite simple.
no scheduling, but I wouldn't.
Happy Days) and by elevators.
stolecoins since requesting an already requested song was a nop.
Once the heads are on the correct cylinder, there may be several
requests to service.
All the systems I know, use Scan based on sector numbers to retrieve
Note that in this case Scan is the same as C-Scan.
Ans: Because the disk rotates in only one direction.
The above is certainly correct for requests to the same track. If requests are for different tracks on the same cylinder, a question arises of how fast the disk can switch from reading one track to another on the same cylinder. There are two components to consider.
parallel readoutdisk with with some network we had devised. Alas, a disk designer explained to me that the heads are not perfectly aligned with the tracks.
Homework: A salesman claimed that their version of Unix was very fast. For example, their disk driver used the elevator algorithm to reorder requests for different cylinders. In addition, the driver queued multiple requests for the same cylinder in sector order. Some hacker bought a version of the OS and tested it with a program that read 10,000 blocks randomly chosen across the disk. The new Unix was not faster that an old one that did FCFS for all requests. What happened?
Often the disk/controller caches (a significant portion of) the entire track whenever it access a block, since the seek and rotational latency penalties have already been paid. In fact modern disks have multi-megabyte caches that hold many 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 the disk is the only part of the system to know the true geometry.
Most disk errors are handled by the device/controller and not the
That is, disks are manufactured with more sectors than are
advertised and spares are used when a bad sector is referenced.
Older disks did not do this and the operating system would form
secret file of bad blocks that were never used.
The hardware is simple. It consists of
The counter reload can be automatic or under OS control. If it is done automatically, the interrupt occurs periodically (the frequency is the oscillator frequency divided by the value in the register).
The value in the register can be set by the operating system and thus this programmable clock can be configured to generate periodic interrupts and any desired frequency (providing that frequency divides the oscillator frequency).
As we have just seen, the clock hardware simply generates a periodic interrupt, called the clock interrupt, at a set frequency. Using this interrupt, the OS software can accomplish a number of important tasks.
Time of day (TOD).
The basic idea is to increment a counter each clock tick (i.e., each interrupt). The simplest solution is to initialize this counter at boot time to the number of ticks since a fixed date (Unix traditionally uses midnight, 1 January 1970). Thus the counter always contains the number of ticks since that date and hence the current date and time is easily calculated. Two problems.
Three methods are used for initialization. The system can contact one or more know time sources (see the Wikipedia entry for NTP), the human operator can type in the date and time, or the system can have a battery-powered, backup clock. The last two methods only give an approximate time.
Overflow is a real problem if a 32-bit counter is used. In this case two counters are kept, the low-order and the high-order. Only the low order is incremented each tick; the high order is incremented whenever the low order overflows. That is, a counter with more bits is simulated.
Time quantum for Round Robbin scheduling. The system decrements a counter at each tick. The quantum expires when the counter reaches zero. The counter is loaded when the scheduler runs a process (i.e., changes the state of the process from ready to running). This is what I (and I would guess you) did for the (processor) scheduling lab.
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 a signal at some future time (the Unix
alarm system call).
The system also on occasion needs to schedule some of its
own activities to occur at specific times in the future
(e.g., exercise a network time out).
The conceptually simplest solution is to have one timer for each event. Instead, we simulate many timers with just one using the data structure on the right with one node for each event.
The objective is to obtain a histogram giving how much time was spent in each software module of a given user program.
The program is logically divided into blocks of say 1KB and a counter is associated with each block. At each tick the profiled code checks the program counter and bumps the appropriate counter.
After the program is run, a (user-mode) utility program can determine the software module associated with each 1K block and present the fraction of execution time spent in each module.
If we use a finer granularity (say 10B instead of 1KB), we get increased accuracy but more memory overhead.
At each key press and key release a
scan 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.
There are two fundamental modes of input, traditionally called
raw and cooked in Unix and now sometimes call
canonical in POSIX.
In raw mode the application sees every
character the user
Indeed, raw mode is character oriented.
All the OS does is convert the keyboard
characters and and pass these characters to the
Cooked mode is line oriented. The OS delivers lines to the application program after cooking them as follows.
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).
Whenever the mouse is moved or a button is pressed, it sends a
message to the computer consisting of Δx, Δy, and the
status of the buttons.
That is all the hardware does.
Issues such as
double click vs. two clicks are all handled by
In the beginning these were essentially typewriters (called
glass ttys) and therefore simply received a stream of
Soon after, they accepted commands (called
that would position the cursor, insert and delete characters,
The End: Good luck on the final