DRAM SSD vs. DRAM-less SSD: Performance, Latency, and Application Guide

What Does DRAM Do in an SSD?

DRAM, or Dynamic Random Access Memory, serves as high-speed working memory for the SSD controller, primarily storing FTL mapping information and metadata for faster access.

An SSD relies on the Flash Translation Layer, or FTL, to manage the mapping between the logical addresses used by the host and the physical locations of data in NAND flash. With dedicated DRAM, the controller can retrieve mapping information more quickly, reducing the need for additional NAND accesses and lowering lookup latency.

DRAM performs several key functions in an SSD:

• Storing the FTL Mapping Table
  

  The host accesses data using Logical Block Addresses (LBAs), while the SSD controller maps those addresses to physical locations in NAND flash.The FTL maintains this mapping so that the controller can locate and manage the requested data efficiently.Storing frequently used FTL mapping information in DRAM shortens lookup time.This advantage becomes more significant when the system processes large numbers of small files, random read and write operations, or multiple concurrent commands.

• Accelerating Data Access Scheduling
  

  In I/O-intensive workloads with frequent random access, DRAM helps the controller manage command queues and schedule read and write operations more efficiently.

  By prioritizing read and write operations more efficiently, the SSD can reduce access latency under heavy workloads and maintain more consistent performance.

• Buffering Host Write Data
  

  Depending on the controller and firmware design, host write data may be buffered in DRAM before being committed to NAND.

  This process can improve write efficiency and help maintain stable performance, depending on the controller architecture and firmware design.

• Reducing Write Amplification
  

  Faster access to mapping information can help the controller manage garbage collection and data placement more efficiently. Depending on the firmware, NAND type, over-provisioning, and workload, this may reduce unnecessary data movement and help control write amplification.
  

  
  
  

How Does a DRAM-less SSD Work?

A DRAM-less SSD does not include dedicated DRAM, but it still uses other memory and caching mechanisms to manage data.These may include controller-integrated SRAM, FTL data stored in NAND, SLC cache, and firmware-based management algorithms.

Some DRAM-less NVMe SSDs also support Host Memory Buffer, or HMB. HMB allows the SSD to use a small portion of the host system memory to store frequently accessed management information. This feature requires support from the NVMe SSD, host platform, operating system, and driver.

A DRAM-less architecture can reduce hardware cost and power consumption. Under intensive random I/O or highly concurrent workloads, a DRAM-less SSD may show higher latency or less consistent performance than an SSD with dedicated DRAM. Endurance should be evaluated separately based on NAND type, firmware, over-provisioning, TBW or DWPD, and the target workload.

DRAM SSD vs. DRAM-less SSD