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Introducing computing and IT
Introducing computing and IT

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6.1 Comparing SSDs and HDDs

Rupert’s homework, a table which compares HDDs and SSDs:

Table 2 A comparison of HDDs and SSDs
Hard disk drives (HDDs) Solid-state drives (SSDs)

Rotating platters and mechanical arms mean they have a relatively fragile construction, leading to a greater risk of data loss and hard drive failure when transported.

No moving parts means a more rugged construction and little or no risk of data loss when transported.

When the drive starts, it takes some time to get up to the right rotational speed. This, coupled with the mechanical arm movement means comparatively slow response time and performance.

Nothing spins and there is no mechanical arm movement, which means comparatively fast responsiveness and performance.

Reduced battery life in a laptop because of a relatively high power requirement.

Prolonged battery life in a laptop because the lack of moving parts means a relatively low power consumption, which in turn means less power needed for cooling.

Access time depends on where the data is situated on the disk.

Access time is the same regardless of the location of the data.

File fragmentation can lead to decreased performance.

File fragmentation still occurs, but performance remains consistent.

Relatively slow in terms of booting up, launching and running applications and transferring files.

Relatively fast in terms of booting up, launching and running applications and transferring files.

Relatively cheap per byte.

Relatively expensive per byte.

Bright and early the next morning, Rupert was eager for answers.

“I can see lots of advantages for SSDs – in particular, the fact that they don’t have any moving parts seems to save a lot of problems. But why is their increasing popularity bad news for us?”

“One important factor is that fragmentation is not a performance issue on SSDs,” said Gloria.

“How come, if they have similar logical file structures?”

“Well, the key to that also has to do with the fact that an SSD has no moving parts,” replied Gloria. “Although an SSD provides secondary storage, because it is a solid-state device it is truly random access memory (RAM). That is why the access time for an SSD is the same, regardless of the location of the data, and so fragmentation does not lead to the same problems as for an HDD.”

Activity 17 (exploratory) RAM and defragmentation

After this hint from Gloria, Rupert understood why fragmentation does not lead to increased or inconsistent access times when accessing files on an SSD comparedwith an HDD.

Write a few sentences to explain his reasoning.


When the head is trying to find the different clusters on a highly fragmented HDD, it has to make many movements over the disk, and wait for the disk to spin round so the cluster is positioned under the read head. This leads to longer access times.

Furthermore, files of the same size can have inconsistent access times, depending on how many clusters they occupy on the faster outer tracks of the disk or on the slower inner tracks. Both of these effects lead to a degraded performance when the disk becomes fragmented.

On an SSD, each cluster can be accessed in exactly the same amount of time, so the performance is always exactly the same, regardless of whether the files on the SSD are highly fragmented.