何謂固態硬碟 (SSD)？

Solid-state drives (SSD) are contrasted with hard disk drives (HDD) by the absence of moving parts. Hard disk drives use a series of data storage platters spinning around a spindle to store data. A read/write head is used to access and write data to the platters, but is joined to an actuator arm that moves along an axis attached to an actuator, which itself is composed of many other moving parts. Solid-state drives, however, use a series of NAND flash memory, those that are found in SD cards, to store data. NAND memory is a non-volatile memory that can retain data even without power. A control chip interprets the data stored.

Moving parts handicap the HDD design compared with SSD design. SSDs can withstand shock up to 1500g/0.5ms, whereas HDDs tolerate much less shock, up to 350g/2ms. Also, because of the limitations of moving parts put on the data access and retrieval times, read/write speeds are significantly higher for SSDs over HDDs. Flash memory makes this possible.

HDDs are not useless though. HDDs are generally cheaper, higher capacity, and longer lasting than SSDs. However, SSDs costs are falling thanks to advancements in technology. Most always, SSDs are higher performance, but have a limited life compared with HDDs. SSD flash memory management relies on specialized patterns to ensure its longest life. Because flash memory cells degrade by a fraction of a percent each time they are written to (not so with spinning platters), eventually they fail, and before HDDs do. Again, technology advances are making this less problematic.

In hybrid deployments, SSDs can be used in tandem with HDDs to make an overall higher performing storage system. Through a technique called SSD caching, which relies on the speed of flash memory, an SSD in series with an HDD can cache reads and writes to the HDD. In this configuration HDDs act as capacity.  

Solid state drives owe much of their benefits to how it stores data. SSDs record data on flash memory chips similar to the ones found in removable media. Flash memory is a type of electronically erasable programmable read-only memory (EEPROM), memory chips that retain information without requiring power. Flash memory uses NOR or NAND logic gates to protect data, most commonly NAND is used in SSD and USB flash drives. This is in contrast to HDDs, which store their data on magnetic platers.

In brief, NAND memory stores data in cells by setting a single cell to a charged or uncharged position, representing binary code. Flash memory can be configured as a single-level (SLC) cell format, multi-level cell (MLC) format, and, less commonly, tri-level (TLC) cell format. Each format stacks cells atop each other to increase capacity, but trading-off durability in the process. A controller is responsible for interpreting the cell charges as zeros or ones, and then further communicating with the host computer. Many other features, such as physical and logical interfaces, are standard SATA, PCIe, or NVMe.

SSDs come in many varieties, capacities, and brand names. However, there is a natural technological divide that separates two classes of SSDs. Those drives that are designed for older interfaces, mSATA III, SATA III, and traditional SSDs, and those designed for high bandwidth applications, PCIe and NVMe SSDs.

• PCIe — PCI Express (PCIe) is a low latency, high data transfer bus for connecting peripherals like graphics cards. A PCIe SSD will look like a PCIe card.
• NVMe SSDs — Nonvolatile Memory Express standard (NVMe) offers higher input output per second (IOPS). NVMe uses multiple parallel channels to reach levels of 16 Gbits/s of throughput. Far superior to any other.
• mSATA III, SATA III, and traditional SSDs — Serial Advanced Technology Attachment (SATA) is a storage interface common to HDDs. SATA is slowly being phased out and replaced by NVMe.

The main benefit of SSD is storage access performance within a small form factor. Following those, there are a few other benefits that make SSD attractive.

• Durability — Without moving parts, SSDs are far superior than HDD in durability. Physical shocks may make HDDs skip, or damage them. The same shocks could seem minor for SSDs. This makes them ideal for mobile devices that are in constant jeopardy of being dropped and damaged.
• Performance — SSDs are some of the fastest performance storage mediums. Compared to HDDs, there’s no doubt. Access data on SSDs is near instantaneous, which means faster boot times, application loading, and system response.
• Lightness and Form Factor — SSDs are built lighter and thinner than other hard drives, thanks to flash memory. This, in combination with its superior durability, makes them ideal for many applications. Laptops, smart phones, and many other devices are lighter and slimmer.
• Power Efficiency — Again, no moving parts easily makes SSDs more power efficient.
• Compatibility — SSDs can go into anything that uses a HDD.