Glossary of terms used on this site

SSD specifications and terminology explained / defined.

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Glossaries

Term Definition
NAND Flash

"NAND" gates are inverted AND logic gates. NAND Flash is the physical component on the SSD responsible for storing your data. NAND Flash memory can be seen as sort of an opposite of RAM (Random Access Memory), which—as you all know—is a type of fast, volatile memory used for storing data temporarily. Non-volatile memory is represented in "permanent" storage -- hard drives, solid-state drives, and Flash memory (USB, SD) devices. The term "NAND" is often used as a truncated term to refer to the dies on the SSD (for example, Micron's 128Gb NAND Flash).

Author: Steve Burke
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NAND Types

NAND Flash comes in various types of memory, with the most common being MLC, TLC, and now VNAND (or 3D NAND). In enterprise and server SSDs, SLC is still somewhat prevalent for its endurance and speed. NAND Types define how many voltage levels each cell (SSD) has on the NAND. SSDs are comprised of billions of cells that are capable of storing data (in bits), each of which must store a "voltage level" in order to "check" the data that is stored. NAND Type is fairly directly tied to endurance of the drive.

Author: Steve Burke
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Overprovisioning

Overprovisioning (OP) -- Overprovisioned space is present in all solid-state drives. Overprovisioned space is inaccessible to the user and cannot store data, but still resides in the dies on the SSD. This space is used as a reserve for background tasks like wear-leveling, garbage collection, and as a back-up in the event of catastrophic cell (SSD) failure. For most consumer drives, the user can expect 7% overprovisioned space (if you see a 120GB SSD, it likely has 128GB of capacity with 8GB reserved for OP).

Author: Steve Burke
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Wear-Leveling

Wear-Leveling -- Wear Leveling is a process we describe briefly in the definition of Overprovisioning, and the two are very closely tied (as is Garbage Collection). Wear-Leveling is a necessary process unique to SSDs and Flash NAND; the process ensures equal wear across all NAND cells on the disk, due to the limited nature of Flash.

Author: Steve Burke
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Write Amplification Factor

Write Amplification Factor (WAF) is a multiplier applied to data during write operations. WAF is the factor by which written data is amplified (can be negatively amplified, or compressed) when performing the operation. Writing data to an SSD actively "kills" the SSD by using up some of its Program/Erase cycles. Many controller manufacturers will implement methodologies by which data is written to a factor of <1.0, meaning data is compressed during the write to cause less damage to the drive's endurance.

Author: Steve Burke
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