How much faster is the memory usually than the disk?

Question

IDE,SCSI,SSD,SATA or all of those.

Sezen

cache my data in a temporary file or MySQL memory db? – Sezen

Answer 1

I'm surprised: Figure 3 in the middle of this article, The Pathologies of Big Data, says that memory is only about 6 times faster when you're doing sequential access (350 Mvalues/sec for memory compared with 58 Mvalues/sec for disk); but it's about 100,000 times faster when you're doing random access.

Orlene

Which is one reason that Vista introduced ReadyBoost... even though sequential access on a flash drive is much slower than a hard drive, there are no mechanical, moving parts on the flash drive so random access is just as fast as sequential access. – Swallowtailed

The link I cited said their test was using a freshly-booted machine, to avoid measuring any O/S caching. – Orlene

"on the flash drive random access is just as fast as sequential access" -- the link I cited says that it's 10,000 times slower. – Orlene

Erhm, I don't know how they tested this exactly, but RAM access in my system is in excess of 26 GB/s, 19 times faster than mentioned here. Also, the tested disks are not exactly a typical setup, unless we're talking servers specifically. – Laconism

tomshardware.com/picturestory/511-4-memory-scaling-ddr3.html says about 100 GB/sec for DDR3 memory. That ACM article was measuring on a server with 64 GB RAM (so I suppose possibly not the fastest/most expensive type of RAM). – Orlene

I don't know about 100 GB/s... I have 1800 MHz DDR3 after all. Still, it's a huge difference. To be fair, my sequential disk access is slightly faster than these figures as well, but only by a small amount (270 MB/s or so) – Laconism

The link is five years old, but wow. If this is accurate its probably the least understood characteristic of computers. – Chubby

Random vs. sequential access is a crucially important distinction here. Even sequential disk access is faster than random RAM access. This highlights the difference between latency and throughput. Sadly, this topic is not widely understood. It should also be noted that CPU registers are between 1 and 1,000 million times faster than some of the fastest SSDs. – Bolshevik

Answer 2

It's not precisely about SCSI drives, but I think that the Latency Numbers Every Programmer Should Know table could assist you in understanding the speed and the difference between different latency numbers, including storage options.

Latency Comparison Numbers (~2012)
----------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy             3,000   ns        3 us
Send 1K bytes over 1 Gbps network       10,000   ns       10 us
Read 4K randomly from SSD*             150,000   ns      150 us          ~1GB/sec SSD
Read 1 MB sequentially from memory     250,000   ns      250 us
Round trip within same datacenter      500,000   ns      500 us
Read 1 MB sequentially from SSD*     1,000,000   ns    1,000 us    1 ms  ~1GB/sec SSD, 4X memory
Disk seek                           10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
Read 1 MB sequentially from disk    20,000,000   ns   20,000 us   20 ms  80x memory, 20X SSD
Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms

Here is a great visual representation that will help you to better understand the scale: https://people.eecs.berkeley.edu/~rcs/research/interactive_latency.html

Tenderfoot

Answer 3

2016 Hardware Update: Actual read/write seq throughput

Now the Samsung 940 PRO SSD

• reading at 3,500 MB/sec
• writing at 2,100 MB/sec

Ram got faster too

• reading at 61,000 MB/sec
• writing at 48,000 MB/sec..

So now using this metric, RAM looks to be 20x faster than the stuff around when @ChrisW wrote his answer, not 100,000. And, SSDs are 10 times faster than RAM was when he wrote this question.

An important consideration is that we're only measuring memory bandwidth not latency.

Shumpert

Answer 4

Random Access Memory (RAM) takes nanoseconds to read from or write to, while hard drive (IDE, SCSI, SATA that I'm aware of) access speed is measured in milliseconds.

Pierson

Answer 5

RAM is 100 Thousand Times Faster than Disk for Database Access from http://www.directionsmag.com/articles/ram-is-100-thousand-times-faster-than-disk-for-database-access/123964

Nugget

Answer 6

Accessing the RAM is in the order of nanoseconds ( 10e-9 seconds ), while accessing data on the disk or the network is in the order of milliseconds (10e-3 seconds).

from Node.JS Design Patterns

Fortuneteller

Can you provide link to the quote ? – Shannan

Answer 7

HDD is significantly slower than RAM due to its reliance on physical movements.

As extensively discussed in this article the speed of HDD can vary much compared to RAM in different scenarios. The speed difference can range from 5 times slower to as much as 80,000 times slower or even 500,000 times slower in certain cases. On average, noting that HDD is 100,000 times slower is a reasonable estimation.

When it comes to handling large datas, HDD can perform up to 80 times slower than RAM. In the case of handling small datas and fast sequential allocations, HDD can be as much as 15,000 times slower, and when performing parallel operations, the speed difference can reach up to 500,000 times slower or even more.

Tribade