Wear Leveling - Samsung 840 White Paper

around old data. Once a block is full of pages that all contain invalid data, that block is considered free and may be erased.
The TRIM command is sent to the SSD controller automatically by the OS every time it deletes a file. As it requires OS
support, not all users will be able to use native TRIM functionality. On PCs, TRIM is supported in Windows 7 or later. On
Macs, TRIM is only supported for Apple's OEM SSDs and is not supported for Samsung's (or any other manufacturers')
aftermarket SSDs. Users of older Windows operating Systems (Windows XP, Windows Vista) may use Magician's built-in
"Performance Optimization" feature to manually pass the TRIM command to the SSD on demand (or via user-specified
schedule).
Bad Block Management & Error Correcting Code (ECC)
In addition to maintenance at the drive level, the SSD must also perform maintenance at the chip level. In every NAND
cell, each page contains a few extra bytes of extra capacity that the SSD controller uses to store a "parity bit." Error-
Correcting Code (ECC) uses this parity bit to compensate for other bits that may fail during normal operation of the drive.
When the controller detects a read failure, it will invoke ECC to try and recover from it. If recovery is not possible, the
firmware's bad block management feature will retire the block and replace it with one of several free "reserved blocks."
"Bad blocks" can be made during read, program, or erase operations and are actively managed to guarantee expected
SSD performance.

Wear Leveling

NAND flash memory suffers from one final limitation: each cell has a finite lifespan and can only withstand a limited
number of program/erase cycles (called P/E cycles). The specific amount of P/E cycles depends on the process
technology (e.g. 27nm, 21nm, 19 nm, etc.) and on the program mechanism (e.g. SLC, MLC). In order to overcome this
limitation, the SSD firmware employs a wear-leveling algorithm that guarantees that write operations are spread evenly
among all NAND cells. Using this technique, no single cell should be unduly stressed and prematurely fail. If too many
cells were to fail, the entire block would have to be retired as just discussed above. There are only a limited number of
reserved blocks, however, so this event should be avoided to prolong overall drive life.
Wrap Up
Fortunately, all of the above procedures (with the exception of TRIM if you're using an older Windows OS) happen
transparently and without action on behalf of the user. While specific implantation will vary, most modern SSDs include all
of these features. In fact, without features like Wear Leveling and ECC (to extend drive life and protect data integrity) and
TRIM and Garbage Collection (to maintain SSD performance), SSD quality and user experience would suffer.
Why Samsung?
Maintenance procedures like wear-leveling and Garbage collection, which are created to overcome the unique properties
of NAND flash memory, work together to help ensure that your SSD performs well over extended use. Together, these
algorithms actually increase write activities to the NAND, which reduces overall lifespan. Thus, the key in designing a
great SSD is finding the optimum balance among lifespan, performance, and reliability. As the #1 player in the memory
business for over 20 years and the largest global supplier of SSDs in the preinstalled storage business, Samsung
has unrivaled knowledge of and experience with SSD technology. Samsung's unique, integrated approach to SSD
manufacturing affords it full control of every component. You can trust that Samsung's expertise is safeguarding your
precious data, and your productivity, when you purchase a Samsung SSD.