Q: What old generations of RAM memory are the same speed as drives?

A: RAM, of any generation, has always been much faster than hard drives or SSD drives, and RAM continues to be faster.

There are several reasons.

Bus access: RAM is on a bus that is much ‘closer’ to the CPU than drives, and with more lanes. The CPU can access RAM through the high speed motherboard connections, whereas hard drives have to be accessed through a SATA or IDE or PCI-e controller, which imposes limits to bandwidth.

Latency: Latency is the time lag between when a request is made and the first data is read. RAM has latency measured in nanoseconds. Drives have latency measured in milliseconds

Going back 20 years, from PC-66 SDRAM to today’s DDR4, RAM latency has always fallen into the range of 8 to 20 ns. As RAM speeds have increased from 66 Mt/s to 2400 Mt/s the latency has increased. Since latency is measured in clock ticks, PC100 runs at 100 MHz clock speed, at latency 2 that equals 20 ns. DDR4-2400 2400 MTs RAM runs at 1200 MHz clock speed so at CAS latency of 16 that equals 13.3 ns.  CAS latency – Wikipedia

Hard drive latency is the average time that the drive needs to process the request, move the heads, and wait for the platters to spin around to the correct block of data. A typical hard drive would have a combined seek time and rotational latency of about 5 ms. or 5,000,000 ns. That’s 250,000 times slower than RAM latency.

SSDs have a very short latency, one of their big advantages over hard drives. In the article below, the read latency was .031 ms. But that is still 31,000 ns, 1,550 times slower than RAM. Sadly SSD’s write performance is much worse than their read performance.

SSD Throughput, Latency and IOPS Explained – Learning To Run With Flash | Page 2 | The SSD Review

Hard disk drive performance characteristics – Wikipedia

Throughput: Drives are also limited in their bandwidth. SATA hard drives can output between 150 and 210 MBps SATA SSD drives can output up to about 540 MBps. Writing speeds will generally be lower. DDR4–2400 RAM memory can output 19.2 GBps or 19,660 MBps, That’s 36 times the bandwidth of the fastest SATA SSD.

PCI-e M.2 NVMe SSD drives can achieve up to 2,800 MBps read speeds (writes slower). Still 7 times less bandwidth than contemporary RAM.

What are the data transfer rates for DDR, DDR2, DDR3 and DDR4?

So: The slowest RAM of the past 2 decades has never been as slow as the fastest drives of today.

Q&A is a compilation of questions we have answered on the Web in a variety of venues

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