Why Power Loss Protection (PLP) is important for Enterprise SSDs

Introduction

Enterprise SSDs differ from consumer SSDs in speed, durability, reliability, and efficiency. A key feature that ensures the reliability of these drives is Power Loss Protection (PLP) which prevents data corruption or loss during unexpected power outages.

This article explains how PLP works, with a focus on capacitors and firmware, and its importance in industries where data reliability is critical.

Why you need a protection against power loss

Lets first see how SSDs write data using a combination of fast volatile memory (DRAM) and non-volatile NAND flash memory to store data and what would a power loss during this process do. Here’s the process of how data is typically written in an SSD:

1. Data written to DRAM and FLT update : When the SSD receives new data, it is first written to the DRAM cache, which is much faster than NAND flash. The DRAM temporarily holds the data, allowing the SSD to handle write requests quickly. The DRAM also stores the Flash Translation Layer (FTL), a mapping table that links logical addresses from the host system to the physical addresses on the NAND flash.
2. Data Transferred from DRAM to NAND Flash: After the data is held in the DRAM, it is gradually transferred to the NAND flash memory, which is non-volatile and can retain data even when the SSD is powered off. The SSD controller manages this process, ensuring that data is written to NAND in an efficient manner.
3. Flash Translation Layer (FTL) Update : The FTL, stored in the DRAM, is updated with the new physical address on NAND

If a power failure occurs during the data writing process, all data currently held in the volatile DRAM cache is lost immediately. Furthermore, even if the data has been transferred to the NAND flash, if the Flash Translation Layer (FTL) hasn’t been updated, the SSD won’t be able to correctly locate or retrieve the data, effectively leading to data loss or corruption.

This is what PLP prevents by ensuring that any in-transit data is either written to non-volatile NAND flash storage or safely discarded in a controlled manner. PLP works by utilizing hardware components like capacitors and firmware algorithms that detect power loss and ensure that data is securely flushed to storage before the SSD powers down.

A combination of Hardware PLP and firmware PLP works best

Firmware is software embedded in the SSD that helps manage data, while hardware is the actual physical components of the drive. In enterprise-grade SSDs, both hardware and firmware PLP are typically used together to maximize reliability.

Hardware-Based PLP:

Hardware-based PLP uses capacitors to provide temporary power to the SSD during a sudden power loss. When the SSD detects a power failure, these capacitors supply enough power for a brief period (milliseconds) to allow the SSD to:

1. Flush the cache: The data stored in the volatile DRAM (cache) is written to the non-volatile NAND flash storage.
2. Update the mapping table: Known as the Flash Translation Layer (FTL), this table maps logical data addresses to physical locations on the drive, ensuring that after a restart, the SSD knows where the data resides.

Capacitors, which are compact and maintenance-free, are ideal for providing quick power discharge in such short outages. Depending on the operating temperature requirements, either tantalum or aluminum capacitors are typically used to provide the necessary energy for this temporary power backup.

Firmware-Based PLP:

Firmware-based PLP focuses on rebuilding the SSD’s mapping table after a power loss. Here’s how it works:

1. The SSD's mapping table is stored in flash memory but frequently updated in DRAM during operation.
2. When power is restored, the firmware uses stored metadata, such as tags and spare bytes (with logical block addresses, ECC, etc.), to rebuild the mapping table, ensuring the SSD can recover the correct data addresses.
3. This approach reduces the risk of data corruption and ensures the SSD is usable upon reboot. Firmware PLP is especially useful in environments where SSDs are used in RAID arrays or share storage pools.

In summary, hardware-based PLP ensures data is safely written during the power-down process using capacitors, while firmware-based PLP focuses on data recovery and maintaining mapping table integrity upon restart.

How PLP Works in SSDs

Capacitors are the core component of the PLP system in SSDs. Here's how PLP is implemented:

1. Capacitors as Power Backup: PLP-enabled SSDs use capacitors to store extra power. During power loss, these capacitors provide just enough power to complete pending write operations, typically within a few milliseconds.
2. Data Flush to NAND Flash: During normal operation, data flows from the SSD’s DRAM cache to the NAND flash storage. In the event of a power loss, PLP systems instantly trigger a data flush process. The SSD’s firmware detects the loss of power and stops accepting new write commands, focusing instead on flushing any data in DRAM to the more permanent NAND flash. Capacitors provide just enough power to complete this operation, ensuring no partial writes occur.
3. Voltage Detection: PLP-capable SSDs are equipped with voltage detectors that monitor power levels. When the voltage drops below a certain threshold (typically around 4 volts), the capacitors activate, providing power for the SSD to finish pending write operations. This process is seamless, preventing the sudden interruption of data operations.
4. Safe Shutdown: Once the data is successfully written to the NAND flash, the SSD performs a graceful shutdown, ensuring that file systems and other critical system structures remain intact. This orderly shutdown prevents data corruption that could render the SSD or stored files unusable after a power outage.
5. Firmware-Based Protection: PLP systems also leverage firmware-based techniques to enhance protection. Features like Auto Idle, data journaling, and power loss event detection further minimize the risks of data corruption. Firmware detects the power loss event and immediately takes appropriate actions, such as flushing data from DRAM or engaging built-in RAID-like protection to recover from partial writes.

Industries That Benefit from PLP in SSDs

PLP is particularly valuable in sectors where even brief periods of downtime or data corruption can have serious financial, legal, or operational consequences. Also in write-heavy environments like data centers and cloud services, PLP is important to mitigates the risk of data loss.

1. Data Centers: Downtime or data corruption can be costly for data centers. PLP ensures that data is preserved during power outages, preventing data loss and reducing downtime in these mission-critical environments.
2. Financial Services: The financial sector relies on the accuracy and availability of data. Power loss without PLP could result in the loss of critical transactional data, leading to financial losses and legal ramifications. PLP-equipped SSDs prevent these risks.
3. Healthcare: Healthcare organizations handle sensitive patient data that must be preserved and protected under regulatory requirements. PLP helps prevent the loss of medical records and ensures that data integrity is maintained, even in the event of a power outage.
4. Government and Public Services: Government agencies often handle vast amounts of critical data. Power loss during important operations could result in the loss of vital information. PLP ensures that this data remains safe and accessible in any situation.

Conclusion

Power Loss Protection (PLP) in SSDs is essential for industries and businesses where data integrity, reliability, and uptime are critical. PLP-equipped SSDs provide a safeguard against unexpected power loss by utilizing capacitors and firmware to complete write operations before the drive shuts down. This ensures that data remains intact and available, preventing corruption, reducing recovery times, and maintaining business continuity.