问题
IDE,SCSI,SSD,SATA or all of those.
回答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.
回答2:
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.
回答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.
回答4:
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
回答5:
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
回答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
来源：https://stackoverflow.com/questions/1371400/how-much-faster-is-the-memory-usually-than-the-disk