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

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

Below are the access times and read / write speeds 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 is the difference between read / write speed and access time? How are they related?

Can you say that RAM is ~1,000x

faster than SSD, and SSD is ~100x

faster than HDD, and therefore RAM is ~100,000x

faster than HDD?


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2 answers

Access time or latency is the amount of time the system waits from a request until data begins to flow. Read and write speed is the amount of data transferred per unit of time. Typically read and write speeds are different for the same device.

These tests are directly related to the technology adopted by each of them. On physical disks (HDD), the read / write speed directly depends on the rotation speed, and the access time is related to the head movement.

In SSD storage, access speed and time are related to internal components and chip organization. An SSD uses multiple flash memory chips that have natural specific access times and storage speeds. The access time also depends on the controller that shares the data across these chips.

RAM modules use dynamic chips (DRAM) which are very fast in speed and access time. The speed is influenced by the chip, but it also depends on the PCB design and the module data bus. Access time, in a way. limited by the refresh rate of the chip.

There is another type of memory called static RAM (SRAM). SRAM uses a much more expensive technology than DRAM, which limits its capacity, but much faster than DRAM. It is used in the processor cache.

Comparing these technologies, it's safe to say that RAM is much faster than SSDs and that SSDs are much faster than HDDs in general. Putting a number is not easy because technology is advancing and each generation of products improves its performance. In addition, server applications have much better performance than consumer products.



Those that seem slightly overpriced, but they are in the ball park. Read and write speeds using a shared file system will be much slower. If you're interested in easy-to-use benchmark utilities, download the memtest86 ISO and that will tell you the actual raw data throughput of RAM. ArgusMonitor for Windows is demo software but will give you raw data hard drive speed.

I saw roughly 20GB / s on average if I am not mistaken for 800MHz DDR2 RAM using 90-130MB / s raw data on a SATA3 hard drive. I didn't have the finances to test SSDs yet, but I've seen them claim to be on average about two to three times faster than a SATA3 hard drive.

Access times are similar to seek times. On a platter hard disk, rotations have to be made and the head has to move to the data retrieval (seek) position and this takes maybe 1-8 milliseconds, which is similar to latency. The solid state access times are relative to what you mentioned and the RAM is slightly less than your estimate, at about 10-15 nanoseconds from the time of the request until the data is received. <Information about RAM



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