Buzz
When you purchase a high-end SSD for your PC, you often notice that the actual capacity displayed on the computer is lower than the advertised capacity. For example, an 8TB SSD may only show around 7TB when installed in a PC. Why is there this difference, why is the SSD capacity lower than advertised?Reason 1: Different measurement methods between Windows and manufacturers
Most manufacturers use the following: 1 kilobyte = 1,000 bytes, 1 megabyte = 1,000 kilobytes, 1 gigabyte = 1,000 megabytes, and 1 terabyte = 1,000 gigabytes. However, Windows follows the binary standard, meaning 1 kilobyte = 1,024 bytes, 1 megabyte = 1,024 kilobytes, 1 gigabyte = 1,024 megabytes, and 1 terabyte = 1,024 gigabytes.Reason 2: Overprovisioning
Before moving forward, let's take a moment to review how SSDs operate.The SSD you're looking at is essentially a series of memory chips bundled together into a drive. Each memory chip organizes data into 'pages.' Some manufacturers may use 2K, 4K, 8K, or 16K per page depending on their design.Multiple pages are organized into a 'block,' typically ranging from 128 to 256 pages per block. This means the size of each block can vary from 256KB to 4MB.
The issue lies in the fact that when an SSD deletes data, it only deletes one block at a time. Writing new data poses no problem, as you simply overwrite. But what about updating existing files or overwriting old ones? Most operating systems handle this by: copying the data of the entire block into RAM, erasing this block, and then writing the new content along with any updates.If the drive is full and there are no empty pages left, the SSD must scan the drive to find blocks marked for deletion (but not yet truly deleted). Only then does the SSD actually erase these blocks and write new data. This is why SSDs slow down over time if not given much free space (as the SSD must rescan the entire process). SSDs have a process called garbage collection, which helps minimize the impact on write performance and the lifespan of the SSD by offloading some tasks to run in the background.To ensure swift data writing processes, an SSD needs to allocate some empty blocks for this purpose, known as overprovisioning. Additionally, overprovisioning helps SSDs deal with the issue of potentially damaged blocks (as there are still reserves) as well as serving the garbage collection process.According to information from Samsung and Seagate, the level of overprovisioning used in modern SSDs is approximately ~7%. Some special drives incorporate dynamic overprovisioning technology, adjusting based on the actual capacity users are utilizing.
Occasionally, you might come across SSDs with unusual capacities like 200GB or 240GB instead of the standard 256GB. These are essentially the same drives, but the manufacturer has deducted a portion of the capacity for overprovisioning. Thus, a 200GB drive effectively becomes a 256GB drive with 28% overprovisioning, a 240GB drive has 7% overprovisioning, while a 256GB drive has 0% overprovisioning.
Reason 3: What drive format is used
Taking the example of Windows 10's NTFS format, it allocates additional space for 'overhead,' which is the capacity reserved for the operating system to manage the drive and partitions. This includes elements like the Master File Table (a small database to locate files), boot table, and various other components. However, this portion is typically insignificant.
