Understanding the Internal Structure and Mechanisms of SSDs

Previous articles mainly discussed mechanical hard drives; now we turn our attention to SSDs, which are pure electronic devices based on NAND flash technology and exhibit much lower access latency than HDDs, especially in random I/O scenarios.

Mechanical hard drives store data magnetically, while SSDs use flash memory similar to USB sticks; this fundamental technology difference leads to vastly different internal structures, illustrated in Figure 1.

Figure 1: Comparison between mechanical hard drives and SSDs.

Unlike the stack of rotating platters in an HDD, an SSD consists of circuits and black storage particles called dies, each built on NAND flash and non‑volatile, meaning data remains after power loss.

Figure 2: Internal structure of an SSD storage die.

Each die contains multiple planes, each plane contains multiple blocks, and each block contains multiple pages. A page is the smallest read/write unit, typically sized 2 KB, 4 KB, 8 KB, or 16 KB.

SSD logical sectors

Historically, operating systems have been built around a 512‑byte sector concept. Modern HDDs use 4 KB physical sectors but maintain logical sectors for compatibility. SSDs follow the same approach: although physical pages range from 2 KB to 16 KB, a logical sector layer is added for compatibility.

The SSD controller logically divides the whole drive into sectors and addresses them using LBA (Logical Block Addressing). When data from certain sectors is requested, the controller consults an LBA map table to locate the corresponding physical page, as shown in Figure 3.

Figure 3: Mapping between logical sector addresses and physical pages.

However, the smallest read/write unit for an SSD is a page; it cannot read or write at the sector level.

SSD cells – flash memory types

Each page consists of many flash memory cells. Modern flash cells come in three main types: SLC, MLC, and TLC.

Figure 4: Types of flash memory cells.

In SLC, each cell stores a single bit (two voltage states). MLC stores two bits per cell by using four voltage states, while TLC stores three bits per cell using eight voltage states. More voltage states increase capacity but also require more precise timing, longer access times, and higher error rates.

From a performance and reliability standpoint, SLC is the best, while TLC offers the highest capacity at lower cost. This explains why industrial‑grade SSDs (which often use SLC or MLC) are more expensive than consumer laptops that typically use TLC.

Figure 5: Comparison table of flash cell types.

Currently, TLC is the most common flash type due to its low price and large capacity.

Thought exercise

If an SSD has a page size of 4 KB and a file is 16 KB, will the file reside in a single storage die or across multiple dies?

To visualize SSD logical structure, see Figure 6.

Figure 6: Distribution of a 16 KB file within an SSD.

If the file were written to a single die, only one flash channel would be used during reads, limiting speed. Distributing the 16 KB across four adjacent dies (4 KB each) allows multiple flash channels to operate in parallel, improving throughput; therefore, real SSDs spread data across multiple dies.