Hard disk drives
The most common storage type is a magnetic spinning platter. It is known as non-volatile storage because even without power the data persists in the system. It also supports random access retrieval, meaning that unlike tape data it does not need to be rewinded or moved forward.
They have a lot of moving parts, such as platters, actuator arms, etc. Any of these parts can break and also limit the access speed.
The rate of speed changes the latency of the drive:
| RPM | Latency |
|---|---|
| 15,000 | 2 ms |
| 10,000 | 3 ms |
| 7,200 | 4.16 ms |
| 5400 | 5.55 ms |
A simple formula to guesstimate the latency to rotational speed of HDDs is 30000 / Speed. |
Solid State Drives
SSDs have no moving parts, and are non-volatile memory. Their biggest advantage is that they are much faster than spinning drives.
One of the key factors that affects drive performance is the interface: SATA interfaces are common, so it is easiy to upgrade from HDD to SSD.
mSATA connectors are mini-SATA, used in laptops and mobile devices. This was briefly used, but not as common since they were very quickly replaced by m.2 interfaces.
AHCI vs NVMe
One of the issues with SATA connections is that they were designed for HDDs, and use AHCI (Advanced Host Controller Interface) to move drive data into RAM. Revision 3 of this SATA standard, for example, can move up to 600 MB/s.
SSDs require a faster communication method, however, since their performance is much faster. This is why we invented Non-volatile Memory Express (NVMe), which has lower latency and higher throughput.
The advantage of NVMe and m.2 connections is that they are smaller, don’t require extra power sources, and can push 4 GB/s+ when using a PCIe bus connection.
Note
M.2 does not guarantee NVMe. It is possible to have an M.2 interface that uses AHCI.
Note that there are different connectors: B key, M key, or both. This is simply the copatibility of the slot key/space of the NVMe drive connector to the motherboard.
Flash drives
Flash memory - technically EEPROM (electrically erasable programmable read-only memory) - is another type of non-volatile memory. An issue is that it was a limited number of writes - once a threshold of writes has been met, the drive can be read but not written to.
Flash memory
USB Flash drives Secure Digital miniSD MicroSD
Optical drives
Optical drives use a small laser to read colors or bumps on a disct. Comon useS: BluRay, CD-ROM, DVD.