Albretch Mueller wrote: > also, if in order to use RAID 10 you need 4 drives (but the dollar > per Gb is approaching $0.02) and you get 1.5 faster performance, what > is the economy of "bying more RAM" if it is so much more expensive? > > Any comparison on HDD, SSD and RAM including pros and cons which is > worth reading?
First, your measurements were leading you to the wrong intuition. Second: https://www.prowesscorp.com/computer-latency-at-a-human-scale/ excerpt: System Event Actual Latency Scaled Latency One CPU cycle 0.4 ns 1 s Level 1 cache access 0.9 ns 2 s Level 2 cache access 2.8 ns 7 s Level 3 cache access 28 ns 1 min Main memory access (DDR DIMM) ~100 ns 4 min NVMe SSD I/O ~25 us 17 hrs SSD I/O 50-150 us 1.5-4 days Rotational disk I/O 1-10 ms 1-9 months Internet call: San Francisco to New York City 65 ms 5 years Internet call: San Francisco to Hong Kong 141 ms 11 years The scale sets one CPU operation -- doing a math operation, perhaps -- at one second of human time. The latency is the amount of time that elapses between the CPU issuing a request and the answer coming back. Now, let's talk about operations per second. RAM can accept requests perhaps once every 40 clock ticks at a clock speed of 4000MHz, which is to say 100 million requests per second. https://en.wikipedia.org/wiki/Memory_timings A high end NVMe SSD might manage 1 million requests per second. More common is 300,000 or so. A good consumer SSD can handle 100,000 to 150,000 requests per second. A good spinning disk might manage 110 requests per second. Not 110,000, just 110. https://en.wikipedia.org/wiki/IOPS Now, let's say you take 4 spinning disks and connect them in RAID10. That's four disks, A1, A2, B1, B2, where A2 gets a copy of every write to A1 and B2 gets a copy of every write to B1. The computer will split writes across the A and B stacks, so a write will complete in half the time it would to just a single disk. But for reads, the computer can ask both the 1 and 2 disks in each stack, so it can handle twice as many reads as a single disk could in a given chunk of time. But that gives the RAID10 system 220 IOPs, still nowhere near the 100,000 IOPs of a single SSD. Does that all help? -dsr-
