Tech Talk

Power Protection for SSDs

Last updated 14 June 2022

Unstable power sources are a common challenge for Solid State Drives (SSDs) in remote and extreme environments. Unlike HDDs, SSDs do not have mechanical parts, so every component is electronic. The internal data management of an SSD involves many operations working in the background, and every power loss or failure, no matter how short, can mean undue interruption of those operations, which could affect drive performance and the loss of critical data.

Data is typically written on the SSD in this order:

  1. The Host sends data to the SSD.
  2. The SSD Storage Controller acknowledges that data has been received.
  3. If the SSD does not have a DRAM cache, the Storage Controller moves the data to the NAND flash memory, which is non-volatile memory, so the data is safely stored even when power is terminated. If the SSD has a DRAM cache, the data is moved there first. This is volatile memory, so if power is terminated, the data is lost.
  4. The Storage Controller decides when to "Flush" the data from the DRAM cache to the NAND flash memory.

In a normal power shutdown, all data from the host is successfully transmitted to the NAND flash memory. Even if power is cut off, data remains there but when power is terminated before data is moved or flushed into the SSD, the data may be corrupted or lost.

This is where Power Protection comes in, often an optional extra for industrial SSDs but there are product ranges with this feature as standard. depending on the requirement of the application Power Protection could mean the difference between a complete loss of data or ensuring that everything is saved.

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The Four Levels of SSD Power-Loss Protection

Power-Loss Protection (PLP) mechanisms built into modern SSDs can be categorized into four levels:

Damaged Firmware Protection (Level 1). Multiple copies of the firmware (FW) are generated at different times to ensure a backup copy exists in the master firmware gets corrupted. The master FW is always stored on the strongest flash pages and access is limited to reduce the possibility of read disturb, and to ensure both FW and data integrity.

Linking Table Damage Protection (Level 2). A NAND flash device creates a linking table to keep track of the physical flash pages with respect to the logical block addresses from the host. The linking table must be maintained and updated constantly so that the host can always access the most updated data via the two mapping addresses recorded on the flash device and the host.

The linking table is cached first and rewritten to the flash from time to time so that the frequent index updating operation does not slow down the ongoing writing performance. When power disruption occurs while some mapping information is still in the flash cache, the mapping index is damaged, along with the data in the flash mapped by the respective linking table. The worst-case scenario is the destruction of the index table, resulting in the loss of all data in the storage device.

Multiple copies of the linking table are backed up for future restore situations when the linking table is distorted or lost during a sudden power loss, thus eliminating the risk of losing all data in a flash device as a result of losing the linking table.

Page/Block Protection (Level 3). Data written on flash chips is divided into segments of millisecond-long operations. If power loss occurs and the writing process is interrupted at one segment point, for example, the 8th millisecond, data error may occur when the host tries to access the part of the data that was lost during the power loss interruption. Level 3 protection ensures completion of the write command prior to power failure, preventing any data loss.

A storage device without advanced PLP technology offers limited protection and data may be lost when a power failure occurs at a certain programming operation segment.

Last Write Page/Block Protection (Level 4). For SSDs with Level 4 protection, data in transit is secured by providing enough power to complete the last program operation before the power loss, guaranteeing that data is safely written onto the flash chip. In the illustration above, the last millisecond-long program operation is completed, and data is safely stored.

The following table summarizes the various levels of power protection:

 LevelDescription of Power Protection Levels
 4Data-in-Flight, Last Page/Block Protection (for data being written onto the flash)
3Data-at-Rest, Page/Block Protection
2Index Table Protection
Less Advanced1FW Protection

Shutting down safely

The image below shows a typical 2.5" SSD with DRAM and controller at the top, followed by two rows of NAND Flash. The yellow ‘boxes’ at the bottom are the power capacitors that store the power required for the SSD to shut down safely and keep the data from being lost.