Задание 1. Прочитайте и переведите текст

The term "memory" is usually reserved for describing the internal storage of a computer. In its strictest sense it refers to the storage locations that can be immediately addressed by the program counter. They are often referred to as the primary storage while magnetic tape, magnetic diskor diskette, magnetic drumare referred to as the secondary storage. One of the standard mediums for the secondary storage is the floppy disk: a flexible disk of plastic, coated on one side or both sides with a magnetic material. Information is stored in concentric tracks of minute magnetized regions; changes in the direction of magnetization represent binary O's and 1's. The information is written onto the disk and retrieved from it by a recording head that is moved radically across the spinning disk to a particular track. The track in turn is divided into a number of sectors, and as a rule information is written or read one sector at a time. A 'more expensive alternative to the floppy disk is the Winchester disk4, in which the magnetic coating is applied to a rigid aluminum platter. For example, a personal computer Winchester disk unit can have such a capacity that it can transfer data faster than a floppy disk. On the other hand, the Winchester disk is permanently sealed in the drive unit, where as a floppy disk can be removed from the drive and replaced by a fresh disk. A simpler, less expensive secondary memory medium is the audio magnetic-tape cassette. One cassette can store about as much information as a relatively low-capacity floppy disk. The access time to a particular address, or storage location, much longer for tape than it is for a disk because the speed of the tape is much lower than that of a disk and because the information is arrayed in a single linear sequence. An important feature, of all the magnetic secondary-storage mediums is that information is maintained even when the computer turned off.

Task  2. Answer the following questions:

Задание 2. Ответьте на следующие вопросы:

1. What is the capacity of floppy disks and Winchester disks?

2. What are the advantages of floppy disks and Winchester disks?

Task 3. Read the text “The Storage Medium” and discuss the following questions in group:

a) What are the common secondary-storage mediums?

b) Which storage medium does she/he use for program storage?

c) Which storage medium would she/he use if she/he had a personal computer and why?

d) Compare the capacity of the storage medium you are using with the capacity of the described mediums.

Task  4. Find the answers to these questions in the text and table below.

1. Give two reasons why server computers often have connected hard drives.

2. Why is RAID  particularly suited to imaging and scientific work?

3. What is the advantage of using drive mirroring?

4. Where is the backup data stored in a RAID 5 system?

5. Which levels of RAID can reconstruct data lost in failed drives from the backup data spread across the remaining drives in the array?

6. Which level of RAID is the fastest?

RAIDING HARD DRIVES

Server manufacturers connect hard drives to ensure that data is adequately protected and can be quickly accessed. Computer engineers call such an arrangement a redundant array of inexpensive disks (RAID). By arranging drives in sets, users hope to take advantage of the higher seek times of smaller drives. A special hard disk controller, called a RAID controller, ensures о that the RAID array’s individual drives are seen by the computer as one large disk drive.

RAID schemes are numbered, with higher numbers indicating more elaborate methods for ensuring data integrity and fault tolerance (or a computer’s ability to

Task  5. Mark the following statements as True or False:

1. Small disks tend to have lower seek times than large disks.

2. RAID controllers make one large hard disk act like a set of small disks.

3. In RAID systems, one disk is filled with data before the next disk is used.

4. A higher numbered RAID array uses a more elaborate system to protect the integrity of data.

5. RAID 0 provides good data recovery.

6. Small file servers do not usually use RAID level 3.

 LESSON 29

Disk Buffers

Microcomputer systems that process large and complicated files often spend considerable time reading from, and writing, massstorage devices. This operation creates long processing pauses that annoy end users. By simply enlarging the PC-DOS/MS-DOS diskbuffer setting from its normal default value of 2, you can dramatically reduce disk I/O delays. A disk buffer is a block of main memory in which the DOS holds data that is being read from or written to a disk. Each time DOS is requested to read or write a record, it first looks to see whether the sector containing that record is already in a buffer. By increasing the size of the disk buffer, the more likely it is that sought-after data will be in main memory. If it is, then DOS simply transfers the record to the application without the need to read the data from the disk which, of course, saves time. The logical solution would seem to be to move all the data used, by an application into the disk-buffer area. Unfortunately, microcomputer systems are not blessed with huge blocks of main memory and a compromise must be made between the amount of memory used for disk buffering and the main memory required for other system operations. Many factors interact to cause disk delays (including facets of the specific hardware configuration and operating system), they cannot be derived theoretically. Instead, they must be measured empirically using the actual system configuration, operating system and "typical" disk transactions made in actual applications. Measuring something as complicated as the timing of Disk I/O in a microcomputer system requires a system model.

Task 2. Read the text “Disk Buffers” and discuss the following problems in group:

a) In what way can a change in disk buffer size affect disk I/O times?

b) What factors can interact to cause disk delays?