What is the contemporary meaning of primary storage?

Question

I've always associated the term "primary storage" with main memory, so memory which can be accessed directly by the CPU and its of the volatile type. The past few days I've been reading some technical articles/publications where the term "primary storage" seems like it's used as synonym for secondary storage and in the Wikipedia page for Computer data storage it says:

Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage

So, I'm guessing that primary storage ≠ main memory and nowadays it's synonym with secondary storage which persist once the system is shut down. Is that correct ?

Also, one of the articles I was reading was: Deduplication for primary block storage

Another one: iDedup: Latency-aware, inline data deduplication for primary storage

Answer 1

What is the contemporary meaning of primary storage?

I believe it now depends on the context that term is used in.

In the context of computer architecture, your understanding of "primary storage" is correct and still used. There is a hierarchy of storage systems in every computer and it still makes sense to use these well established names when speaking of the entire computer's storage systems.

There is also the context of persistent storage systems, like a data center or a NAS. It also makes sense to classify the drives into a hierarchy based on how they are used and what data they contain. The drives that are used more often (aka the "hot" storage) can be referred to as "primary storage", while the drives containing backups would be considered "secondary storage".

Think of it this way, a computer may have:

• Primary Storage (RAM)
• Secondary Storage (Persistent)
  • Primary "secondary" storage (actively used data)
  • Secondary "secondary" storage (backups of the "primary" drives)
• and so on...

Answer 2

You have to admit that the WikiPedia Online Library has its ways and methods to confuse the readers instead of making their minds clearer. This idea is true for the paragraph that you have mentioned:

"Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage."

The paragraph is not backed up by anything else than a reference. However, the primary storage is the main memory as you have understood it correctly. Their definition for the primary storage appears in the beginning of the section that is ended with that confusing paragraph.

"Primary storage (also known as main memory, internal memory or prime memory), often referred to simply as memory, is the only one directly accessible to the CPU."

So basically the Central Processing Unit has direct access to the primary storage. Its most common form is the Random Access Memory.

The non-primary storage could be seen as the type of storage that is indirectly accessed by the Central Processing Unit through Peripheral Devices. In WikiPedia's view, it has been categorised as secondary storage, tertiary storage and off-line storage.