Glossary

SSD

An SSD, or solid state drive, is a storage device built from flash memory chips and a controller, with no moving parts. It reads and writes data electronically, which makes it far quicker than a hard disk for random access. Its limits are cost per terabyte and wear from writes.

What is inside an SSD?

Two parts do the work. The NAND flash chips hold the data. The controller decides where each piece goes, tracks which pages are current and which are stale, spreads wear evenly across cells, corrects read errors and runs garbage collection in the background. Much of what separates one SSD from another is the controller firmware, not the chips.

The drive also reserves spare capacity the host never sees, so it can retire worn cells and keep room for housekeeping.

How do SATA, SAS and NVMe SSDs differ?

The interface decides how much of the flash the host can reach. SATA and SAS SSDs reuse connections designed for spinning disks, with a single command queue. NVMe drives connect over PCI Express with many parallel queues. The chips are the same type. The NVMe drive simply exposes more of their speed.

Why do SSDs differ so much in price and rating?

Flash type is the main driver. Cells holding fewer bits per cell last longer and write faster; those holding more, as in QLC flash, cost less per terabyte. Enterprise drives are also rated for sustained load, include power-loss protection so in-flight writes are not lost, and carry endurance ratings expressed as drive writes per day. Consumer drives drop some of these features.

Where do SSDs wear and where do they not?

Reads do not wear flash in any practical sense. Writes and erases do, and small random overwrites wear the drive faster than large sequential ones. A drive used to hold data written once, such as an archive or a dataset, barely uses its rating, while one under a journal or metadata store uses much more. The ratings are covered in flash storage endurance, and the comparison with hard disks in flash storage vs HDD.

Frequently asked questions

Do SSDs need defragmenting?

No. Location makes no difference to access time on flash, and rewriting data only adds wear.

Do SSDs lose data without power?

Eventually, if left unpowered for long periods. Charge leaks slowly from the cells, faster on worn drives and in heat.

Is an SSD the same as flash storage?

An SSD is one form of it. Flash also appears in arrays, memory cards and other devices.