For disks/RAM, what is the relationship between access time and read/write speed? [closed]

Question

I'm trying to better understand how relatively fast HDD/SSD/RAM is when it comes to reading/writing bytes.

Here are the access times and read/write speeds I've found from online sources:

Storage | Read/Write speed | Access time
RAM     | 100 GB/s         |   50 ns
SSD     | 0.5 GB/s         |  500 ns
HDD     | 0.1 GB/s         | 5000 ns   

My initial thought was that access time is the time it takes to read 1 byte, but it looks like these numbers don't support that. What exactly is the difference between read/write speed and access time? How are they related?

Is it safe to say that RAM is ~1,000x faster than SSD, and SSD is ~100x faster than HDD, and hence RAM is ~100,000x faster than HDD?

Answer 1

Access time or latency means how long the system wait from request until data starts to arrive. Read and write speed are the amount of data transferred per time unit. Usually read and write speeds are different for the same device.

These benchmarks are directly related to the technology adopted by each one. On physical disks (HDD), the read/write speed are directly affected by the rotational speed and the access time are related to the movement of the head.

On SSD storage, speed and access time are related to chip internals and organization. SSD uses multiple flash memory chips which have a natural specific access time and speed to store data. Access time is also affected by the controller that split data through these chips.

RAM modules use dynamic chips (DRAM) that are very fast in speed and access time. The speed is affected by the chip but also affected by the pcb design and data bus of the module. The access time, in some way. is limited by the refresh rate of the chip.

There is also another kind of memory called static RAM (SRAM). SRAM uses a much more expensive technology than DRAM that limits its capacity but far faster than DRAM. It is used on processors cache.

Comparing these technologies, it is safe to say that RAM is much faster than SSD and that SSD is much faster than HDD in general way. Put in number is not so easy because technology evolves and each generation of products have improvements in its performance. Also server application devices have much better performance than consumer products device.