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What Is Sequential Access?

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  • Written By: John Lister
  • Edited By: Kristen Osborne
  • Last Modified Date: 11 September 2014
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Sequential access is a system by which stored data is accessed in a fixed order. One example of this is an audio or video cassette. The most common use of this system is in computer memory, where it contrasts with the more widely used random access memory.

The phrase sequential access simply refers to the fixed order of access. As well as being fixed, this order is pre-determined and follows a logical progression. Some types of sequential access are unavoidable, such as when playing back a cassette. Other types are a deliberate choice. This can include computer data stored as a linked list, in which each piece of data contains a reference to the following piece.

Sequential access memory is a storage system where the data is stored and read in a fixed order. This contrasts with random access memory, such as that used by computers for storing data to be processed. As a rule of thumb, sequential access is the type of memory more likely to be used for permanent storage and random access memory for temporary storage.

Examples of sequential access memory include older recordable media such as DVDs, CDs, and even tapes. Hard drives are also sequential, rather than random access memory. Random access memory included memory chips found in computers, as well as flash memory that can be fixed inside a device or removable, such as memory sticks or memory cards.

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The main advantage of sequential, as opposed to random access memory, is that it is usually much cheaper to produce. This also makes it possible to get more data capacity given both a set price and a set physical size limit. Sequential access memory is also more commonly used in physical storage forms that are less susceptible to physical damage, including from electrical surges or power loss.

The main drawback of this type of memory is that it usually takes longer to physically access any one piece of data. This is because the computer either has to run through all the data in order until it find the correct piece, like looking for information in a book with no index, or it has to use a seek function that knows where the data is stored but still needs to physically move to the relevant section of the storage device. In contrast, random access memory is set up so that it usually takes an identical time to find any individual piece of data.

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nony
Post 3

@David09 - I do like sequential data access for speed, but honestly I prefer random access for its convenience.

When I open files for random access, I get to use a “seek” operation which lets me jump to anywhere in a file to find stuff. This is good for filtering and things.

However, each seek operation slows the database down, and that’s what makes it slower than sequential access, where the disk just starts at the first location and keeps moving until the end. It’s a tradeoff between speed and convenience, in the end.

David09
Post 2

@SkyWhisperer - I used to mess around with dBase back in the early 1980s. Dbase was a sequential database – basically a bunch of flat file records.

Ordinarily this would mean that the database ought to be slow, since you have to go through each record one at a time.

But one of the ways that dBase overcame that speed limitation was that they had indexes for each sequential access file in the database. The indexes contained the few fields that were really needed to identify a record, like a last name or social security number or whatever. Indexes made the database run a whole lot faster in my opinion.

Since then I’ve migrated over to Microsoft Access, but I think in comparison dBase was actually faster in some cases, especially when used as a back end to Visual FoxPro.

SkyWhisperer
Post 1

I do agree that hard drive storage requires sequential access. However that doesn’t mean that hard drives are always efficient. This is because your data gets scattered on a hard drive.

The reason for that is that it usually allocates blocks of space for each file you store, whether it’s a big file or a small file. If it’s a small file you wind up with a lot of wasted left over space in that block.

For that reason I always make it a point to defragment my hard drive, sometimes twice a month. What that does is take all of the data and squeeze it into all available empty space, stacked close to each other, so that the hard drive head has less distance to travel when it goes from one file to the next. My computer runs a lot faster as a result.

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