Operating Systems – Week 6 Lecture 1

I/O Management and Device Types

Despite the multitude of input/output (I/O) devices that constantly appear (and disappear) in the marketplace and the swift rate of change in device technology, the Device Manager must manage every peripheral device of the system. To do so, it must maintain a delicate balance of supply and demand - balancing the system’s finite supply of devices with users’ almost-infinite demand for them.

This week looks at the Device Manager’s four basic functions:

· Monitoring the status of each device

· Enforcing preset policies

· Allocating each device appropriately

· Deallocating each device at two levels

Although many users may think of an I/O request as an elementary machine action, the Device Manager actually divides the task into three parts, with each one handled by a specific software component of the device management subsystem. 

1) The I/O traffic controller monitors the status of every device, control unit, and channel. This is a job that becomes more complex as the number of units in the I/O subsystem increases and as the number of paths between these units increases. Discuss the tasks that must be performed for each I/O request.

2) The I/O scheduler performs a job analogous to the one performed by the Process Scheduler described in Chapter 4 on processor management – that is, it allocates the devices, control units, and channels. Note that some systems allow the I/O scheduler to give preferential treatment to I/O requests from high-priority programs.  

3) The I/O device handler processes I/O interrupts, handles error conditions, and provides detailed scheduling algorithms, which are extremely device dependent.

Sequential Access Storage Media

The first secondary storage medium used for electronic digital computers was paper in the form of printouts, punch cards, and paper tape. Magnetic tape followed for routine secondary storage in early computer systems.

The figure below is a zoomed in top view of magnetic tape.   

Nine-track magnetic tape with three characters recorded using odd parity. A 1/2-inch wide reel of tape, typically used to back up a mainframe computer, can store thousands of characters, or bytes, per inch. Parity bits are used for error correction in case on of the other data bits is corrupted.

The term “Tape Density” refers to the number of characters recorded per inch. 

Sequential nagentic tape contains an “Interrecord gap (IRG)” which may be a ½ inch gap inserted between each record. The IRG is the same size regardless of sizes of records it separates

IRGs in magnetic tape. Each record requires only 1/10 inch of tape. When 10 records are stored individually on magnetic tape, they are separated by IRGs, which adds up to 4.5 inches of tape. This totals 5.5 inches of tape. Are we wasting tape here?

We can also divide data (records) into blocks. Ultimately we are looking to maximize Transfer rate which equals: (tape density) x (transport speed). Using an “Interblock gap (IBG)” and a ½ inch gap inserted between each block, we have can achieve a better transfer rate than individual records and IRG. The optimal block size is achieved if  the entire block fits in the buffer.

· Two blocks of records stored on magnetic tape, each preceded by an IBG of 1/2 inch. Each block holds 10 records, each of which is still 1/10 inch. The block, however, is 1 inch, for a total of 1.5 inches. Our Tape Density is greater and thus we achieve a greater transfer rate.

· Blocking does have some disadvantages:

· Overhead and software routines needed for blocking, de-blocking, and record keeping

· Buffer space wasted when only one logical record needed

· Note the main disadvantage to sequential media in general is that requested data can be anywhere on the tape and the every ounce of data before the requested data must be analyzed before we even get to our records. The table below shows just how slow it can be to find a file on magnetic tape in worse case scenarios:

Access times for 2400-foot magnetic tape with a tape transport speed of 200 inches per second.

· Note that magnetic tape is not obsolete. In fact magnetic tape is the primary backup solution for most Data Centers in production today. A typical solution may be a robotic library with 1000 tapes within. There may be 20 or so tape drives connected to the library as well. Thousands of servers can connect to such a library over a SAN (Storage Area Network) to perform nightly backups to tape. Tape operators physically remove and reload the library daily so multiple copies of backups are stored (typically both on site and off site in case of a disaster).

Direct Access Storage Devices

Direct access storage devices (DASDs) include all devices that can directly read or write to an arbitrary place in storage. Note that DASDs can be grouped into three categories: magnetic disks, optical discs, and solid state (flash) memory.

Magnetic Disk Storage

Magnetic disk drives, such as computer hard drives (and the floppy disk drives of yesteryear), usually feature one or more read/write heads that float over each surface of each disk.   Disk drives can have a single platter, or a stack of magnetic platters, called a disk pack.

· A disk pack is a stack of magnetic platters. The read/write heads move between each pair of surfaces, and all of the heads are moved in unison by the arm. This is the side view of a typical hard disk drive. 

The top view of an actual disk drive is shown below. On a typical hard disk, the arm moves two read/write heads between each pair of surfaces: one for the surface above it and one for the surface below, as well as one for the uppermost surface and another for the lowermost surface.   

To access any given record, the system needs three things: its cylinder number, so the arm can move the read/write heads to it; its surface number, so the proper read/write head is activated; and its sector number.     See the diagram below for how a disk platter is divided into tracks and sectors.  

· The total time required to access a file on a hard disk is determined by three factors:

o Seek time (slowest) – This is the time required to position the read/write head over the correct track.

o Search time – This is rotational delay – i.e. The time it takes to rotate the DASD until the desired record is under the read / write head

o Transfer time (fastest) – The time to actually transfer the data from the platter through the read / write head and to main memory.

Total Access  Time  = Seek Time + Search Time + Transfer Time

· There is another kind of magnetic drive called the “Fixed Head” magnetic drive.     This type of disk drive contains a read / write head that does not move – it remains in a fixed position over the entire platter.      Because there is no time involved in positioning the read / write head, the Seek time is eliminated, making the total transfer time = search time + transfer time.

· The diagrams below demonstrate how a fixed head drive looks and performs:

· As a disk rotates, Record 1 may be near the read/write head and ready to be scanned, as seen in (a); in the farthest position just past the head, (c); or somewhere in between, as in the average case, (b).

· Access times for a fixed-head disk drive at 16.8 ms/revolution.

· Typical times for a movable head head drive such as a typical hard drive.   

Device Handler Seek Strategies

A seek strategy for the I/O device handler is the predetermined policy that the device handler uses to allocate access to the device among the many processes that may be waiting for it. It determines the order in which the processes get the device; the goal is to keep seek time to a minimum.    There are multiple sscheduling algorithms for optimizing seek time and their main objectives are to  minimize arm movement, minimize average response time, and minimize variance in response time.

First-come, first-served (FCFS) – read / write arm moves from track to track to fulfill requests in the order received.   On average this does not meet the three seek strategy goals as the algorithm typically results in extreme arm movement.

· Shortest seek time first (SSTF) – processes rrequests with track closest to one being served even if the request is not next in line.     This strategy minimizes overall seek time by postponing traveling to out of way tracks and thus reducing total arm movement.

· Other algorithms include “SCAN” which contains multiple variations (LOOK, N-Step SCAN, C-SCAN, and C-LOOK).

· The idea behind Scan Algorithms is to move the arm methodically.     The arm basically always moves as far as it can inward, processing everything along the way.    When it reaches the inner most track, it turns around and reverses direction, again processing everything along it way back outward.

Optical Disc Storage

Advancements in laser technology made possible the advent of CD, DVD, and Blu-ray optical disc storage. Note however, that there are many differences between an optical disc and a magnetic disk, including the design of the tracks and sectors. 

· Optical Disk Design features

o Single spiralling track

o Same-sized sectors: from center to disc rim

o Spins at constant linear velocity (CLV)

o More sectors and more disc data than magnetic disk

· On an optical disc, the sectors (not all sectors are shown here) are of the same size throughout the disc. The disc drive changes speed to compensate, but it spins at a constant linear velocity (CLV). A magnetic disk spins at a constant Angular velocity (CAV) which means it is always spinning at the same speed.

Some of the most important measures of optical disc drive performance are sustained data transfer rate and average access time.

Search Strategies: Rotational Ordering

Rotational ordering is used to optimize search times. Requests are rearranged once read/write heads positioned. This prevents the need to come back to the same track later on if other requests are on the same track or a nearby track.  This reduces time wasted due to rotational delay. Note that only one read/write head can be active at any one time, so the controller must be ready to handle mutually exclusive requests.

CD and DVD Technology

· The figure below demonstrates what the data looks like on the surface of the three mediums listed below. 

· Notice how much more data is packed on a track on a Blu-ray vs a CD. Blu-ray and DVD technology also allow for more than one surface – i.e. the laser can read and right to and from multiple layers within the same “film.” 

To put data on an optical disc, a high-intensity laser beam burns indentations on the disc that are called pits. These pits contrast with the unburned flat areas, called lands. Note that optical discs can have one to multiple layers, allowing the laser to look through upper layers to read those below.

Rewritable discs (classified as CD-RW and DVD-RW) use a process called phase change technology to write, erase, and rewrite data. The disc’s recording layer uses an alloy of silver, indium, antimony, and tellurium. The recording layer has two different phase states: amorphous and crystalline. In the amorphous state, light is not reflected as well as in the crystalline state.

Although DVDs use the same design and are the same size and shape as CDs, they can store much more data.

Components of the I/O Subsystem

Regardless of the hardware makeup of the system, every piece of the I/O subsystem must work harmoniously.

· I/O subsystem: a collection of modules within the operating system that controls all I/O requests.    The diagram below is an overview of the entire I/O Subsystem.

· I/O channel: a specialized programmable unit placed between the CPU and the control units, which synchronizes the fast speed of the CPU with the slow speed of the I/O device and vice versa, making it possible to overlap I/O operations with CPU operations.

· I/O control unit: the hardware unit containing the electronic components common to one type of I/O device, such as a disk drive.

· Disk controller: (disk drive interface) Links disk drive and system bus

The example I/O subsystem above is typical of an enterprise server.    Its configuration contains multiple paths, which increase both flexibility and reliability. With two additional paths, shown with dashed lines, if Control Unit 2 malfunctions, then Tape 2 can still be accessed via Control Unit 3.