Open-channel SSD

Lore & Background

Open-channel SSDs emerged as a design that removes the traditional Flash Translation Layer (FTL) from the SSD controller. The FTL, which maintains a logical-to-physical (L2P) address mapping, is instead stored in host memory and managed by the host CPU. This shift allows the operating system to directly control how data is placed on NAND flash memory, which is organized hierarchically into dies, planes, blocks, and pages. NAND flash imposes constraints: writes must fill full pages, writes must be sequential within a block, and a block must be erased before any page in it can be rewritten. The number of program/erase (PE) cycles is also limited. By integrating L2P management with system software, open-channel SSDs can reduce redundancy between the OS and device firmware, improving performance and endurance. However, open-channel SSDs are not a uniform class; critical characteristics such as the minimum unit of read and write vary between devices, so no single FTL works on all of them.

Reader's Guide

The significance of open-channel SSDs lies in their departure from the traditional SSD architecture, where the FTL is handled by dedicated firmware and DRAM on the device. By moving L2P management to the host, open-channel SSDs enable tighter integration with file systems, allowing the operating system to coordinate data layout, garbage collection, and request scheduling. This can remove conflicts between system software and SSD firmware, smoothing performance and improving endurance. However, this flexibility comes at a cost: a significant amount of host memory and CPU cycles are required. For example, with an average write size of 4 KB, managing the L2P table for a 1 TB SSD consumes almost 3 GB of host RAM. The approach is supported by operating system kernels such as Linux 4.4, which uses the LightNVM interface to access open-channel SSDs following the NVM Express specification. The legacy of open-channel SSDs is that they offer a path for more efficient storage management by eliminating the duplication of effort between the OS and device firmware, though they impose higher demands on host resources.

Did You Know?

• Open-channel SSDs do not have a firmware Flash Translation Layer (FTL) on the device.
• The Linux 4.4 kernel supports open-channel SSDs that follow the NVM Express specification.
• The interface used by the operating system to access open-channel SSDs is called LightNVM.