Why is accessing RAM always faster than accessing Hard Drive?

I am a web developers, but I have a lot of interest in OS, system administration etc. I always hear that RAM is faster, but never really got a chance to ask "Why".

Please help me here. Thanks in advance.

Denny Trebbin's photo

The easy version:

  • HDD is relatively far away from the CPU, connected to system logic board via a somewhat slow SATA connection. SATA 6GB/s is the current standard. HDD's can only read or write but not both at the same time. ​
  • RAM sits very close to the CPU and has a very high bandwidth connection. DDR4 has a throughput of ~40GB/s some graphs RAM's can read/read, read/write and write/write at the same time (dual channel). RAM's can also send very large data at once (double data).

The technical version:

  • HDD's have spinning platters, and a magnetic read/write head needs to reposition every time you want to read/write something. It needs to find the correct places on each platter. HDD's platters are spinning at 5400 rpm, 7200 rpm or 10000 rpm. The magnetic read/write head sits on each side of a platter. Typically an HDD has three platter and six read/write heads. But those heads are fixed to the movement of just one torque engine. Depending on the layout of a filesystem the read/write head can't constantly read from near positions.

  • RAM memory typically modules consists 4 or 8 memory modules. Each module is connected via dual-channel and read/read, read/write or write/write at the same time. At best a RAM memory with eight modules can 16 times read data at the same time or write 16 times at the same time. Each memory module has its layout manager; that knows exactly where the data is. Reading data is only affected by something called CAS timing. For example a DDR4 RAM clocks at 1800 Mhz (1,800 billion clock cycles / second ) and has a CAS timing of 34; means that the RAM module just needs 34 clock cycles to be ready.

skids's photo

As Denny said, prior to SSDs a lot of the delay accessing data stored there was due to the fact that you had to physically move a mechanical apparatus to find the place on the disk where the data was stored, like moving the needle on a record player to find the right song, and then wait for the data to spin around until it was underneath the needle. This was known as "seek time" and dominated when you wanted to access a lot of little things that were spread around across the disk.

SSDs do not have such a head, which is why they are a lot faster for many purposes. However, they do have a "wear-leveling" algorithm that tries to use all the bits on the drive evenly, and can be a source of a (much tinier) amount of delay during writes. SSDs are still also a "block-based" device meaning data is fetched and written in larger batches than with RAM. Again this only slows down scattered accesses compared to reading large files in order.

Even were we to eliminate all that, there is still the fact that drives are usually on a slower, narrower bus located further away from the CPU. The RAM on the mainboard, in turn, is farther from the CPU and often on a slower bus than the RAM in the cache (which is sometimes divided into levels that each of which in turn are closer to the CPU, but smaller than one before it.) Note that electrical signals which carry the bits to the CPU are limited by the speed of light. Light is pretty fast, but even so, in one clock cycle on a 4Ghz CPU it only moves about the length of your finger. So if you read an address out of memory, and then use that address to read something else from memory, you have to make a round trip to RAM to do that, and the number of clock cycles it takes to do that will depend on how far away those bits are stored. This is why, even with RAM, these days we often read blocks at a time (smaller blocks than disk, though) and try to keep data items that get used at the same time time next to each other when we can, to increase the chances that they will be in the same block.

Vasan Subramanian's photo

This is a grossly simplified explanation, but I hope you can relate to it.

Essentially, there are two aspects that determine the speed.

(a) How the bits are stored

RAM uses solid state technology (ie, semi-conductors), and this is super-fast. Reading / writing is as simple as checking / setting the voltage at the storage location. You can say this happens at the speed of light.

Conventional HDDs store information magnetically. To read or write, a magnetic head has to physically move to a position in the magnetic media and do its stuff. Much slower, as you can imagine. You can think of it as a magnetic cassette tape, but whirring much faster.

A Solid State Drive (SSD) uses solid state technology. But it is permanent storage (unlike RAM, which vanishes when power is switched off). Thus, the speed of reading / writing is slower. You can think of it like it has to do more stuff than just set the voltage, so that the state of 0/1 stays permanent.

(b) How the bits are transferred

RAM is connected to the CPU via a super-fast parallel 'bus'. Imagine a bunch of 64 wires going from the CPU to the RAM. In a clock tick, the voltage a the storage location (address) is 'felt' by the CPU, 64 bits at a time. The bus's clock speed can be as fast as 1,800 MHz these days.

For a conventional HDD, a serial cable is used (SATA). Even though SATA can run as fast as 3000 MHz, it still transfers one bit at a time. You can do the math.

SSDs mimic HDDs for transferring data due to convenience (because no one invented special buses for SSDs yet). So, they too use SATA, and thus limited by that speed.

Sukanta Banerjee's photo

CPU only processes data resides in RAM, so whatever be the data need to keep in RAM in order to get processed by the processors.

So the time is taken is as follows

(cache memory->(if data is missing in cache)RAM->(if data is missing in RAM) others(e.g. HDD or EXTD)