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datapatterns [2013/04/20 09:37] (current)
sjoerd created
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 += Data Patterns =
  
 +Here you can find some information with explanation about some commonly used terms in storage terminology.
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 += Sequential =
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 +Refers to reading or writing data records in sequential order, that is, one record after the other. To read record 10, for example, you would first need to read records 1 through 9. This differs from random access, in which you can read and write records in any order.
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 +Some programming languages and operating systems distinguish between sequential-access data files and random-access data files, allowing you to choose between the two types. Sequential-access files are faster if you always access records in the same order. Random-access files are faster if you need to read or write records in a random order.
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 +Devices can also be classified as sequential access or random access. For example, a tape drive is a sequential-access device because to get to point q on the tape, the drive needs to pass through points a through p. A disk drive, on the other hand, is a random-access device because the drive can access any point on the disk without passing through all intervening points.
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 += Random =
 +Refers to the ability to access data at random. The opposite of random access is sequential access. To go from point A to point Z in a sequential-access system, you must pass through all intervening points. In a random-access system, you can jump directly to point Z. Disks are random access media, whereas tapes are sequential access media.
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 +The terms random access and sequential access are often used to describe data files. A random-access data file enables you to read or write information anywhere in the file. In a sequential-access file, you can only read and write information sequentially,​ starting from the beginning of the file.
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 +Both types of files have advantages and disadvantages. If you are always accessing information in the same order, a sequential-access file is faster. If you tend to access information randomly, random access is better.
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 +Random access is sometimes called direct access. ​
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 += Latency =
 +In general, the period of time that one component in a system is spinning its wheels waiting for another component. Latency, therefore, is wasted time. For example, in accessing data on a disk, latency is defined as the time it takes to position the proper sector under the read/write head. 
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 += Seek Time =
 +For disk drives, the terms seek time and access time are often used interchangeably. Technically speaking, however, the access time is often longer the seek time because it includes a brief latency period. ​
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 += Access time =
 +Access time is also frequently used to describe the speed of disk drives. Disk access times are measured in milliseconds (thousandths of a second), often abbreviated as ms. Fast hard disk drives for personal computers boast access times of about 9 to 15 milliseconds. Note that this is about 200 times slower than average DRAM.
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 +The access time for disk drives includes the time it actually takes for the read/write head to locate a sector on the disk (called the seek time). This is an average time since it depends on how far away the head is from the desired data. 
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 +{{tag>​notes storage}}
datapatterns.txt ยท Last modified: 2013/04/20 09:37 by sjoerd