Lösungen für Flash Data Retention & Read Disturbance von Innodisk

Innodisk, Eindhoven, 24.01.2023 - The Solid-State Drive (SSD) has become a staple across many industries. This is especially true for devices designed for harsh environments, as SSDs are generally more robust than other storage media. However, data retention still remains a challenge in many developments.

The quality of a flash cell is not solely defined by the number of program-erase cycles before it can no longer be written, erased, or read. The so-called Data Retention Time is the defined period during which the flash can hold data without refresh. Data Retention describes the ability of NAND flash to preserve data stored over a longer period. “Read Disturb” is a phenomenon affecting neighboring memory cells when a single cell is frequently read. This can also lead to data loss. These effects require special functions to combat data retention issues.

Read disturbances can occur when the same cell in a block is accessed frequently. This happens when a user repeatedly checks specific information on their device, such as a phone number. This is because reading data from a cell causes slight shifts in the threshold voltages of other cells in the same block. Each time data stored on a cell is accessed, a read voltage is sent through all cells in the block. For the cells that the voltage affects, electrons are emitted into the open gate, changing the electrons stored in those cells. While normally harmless, the effects of these voltage shifts are amplified when a single cell is read frequently, and in some cases, data on neighboring cells can become completely inaccessible.

The ability of NAND flash to store electrons is limited, as the stored electrons tend to leak over time. This results in ECC errors and data loss. Several reasons make data storage in NAND flash problematic. Due to the structure of the flash cell, higher temperatures cause data to degrade at very high speeds. With increasing P/E cycles, the cell further weakens, leading to reduced data retention capacity.

Since both programming and erasing NAND memory cells generate high electric fields, the oxide layer around the floating gate can degrade over time as the number of P/E cycles increases. This is especially true for the tunnel oxide layer beneath the floating gate. Using 100-bit errors as a standard for data retention, the retention period after 2200 P/E cycles is about 48 days, but after 3200 P/E cycles, it drops to around 16 days.

Innodisk's iRetention 2.0 technology consists of preventive and corrective features to combat the shortcomings of flash memory in data retention and read disturbance issues.

Dynamic Micro-Loading periodically and sequentially selects a block and randomly reads a page. This micro-load tests all cells within the block once, balancing out the negative effects of storage. Similarly, Smart Read performs a read task across the entire SSD during boot-up to ensure better data integrity in future read cycles.

Dynamic Scan regularly scans the SSD to detect potentially faulty data and performs the refresh process, which involves flushing and recharging pages to extend the lifespan of each data block.

Together, these technologies virtually eliminate the risk of SSD data loss and enhance data retention for Innodisk products.

Learn more about challenges and solutions for data storage in the whitepaper "Modern Solutions to Flash Data Retention & Read Disturbance".