DRAM SSDs are known for their high performance and reliability. On the other hand, DRAM-less SSDs are a budget-friendly option for users who want to prioritize cost over peak performance. These people are generally good with slightly lower speeds in exchange for savings. Low-end systems can work pretty well even with the DRAM-Less SSDs. The selection also depends on the purpose of your drive or what you really want to do on your computer.
DRAM SSDs will have their own dedicated DRAM chip on the SSD itself. It will serve many purposes including read/write caching, buffering, table management, garbage collection, etc. However, in the DRAM-Less SSDs, you won’t see any DRAM chip on the drive.
In DRAM-Less SSDs, the drive will either use a small part of the primary memory i.e. NAND Flash memory or HMB (Host Memory Buffer). In HMB, the drive will use your system’s RAM for caching purposes but the performance will be compromised as compared drive’s own DRAM.
DRAM SSDs have undeniable benefits while DRAM-Less SSDs can also get most of the work done without many issues. Unless you are a performance seeker or have some heavy workloads on your drive, the DRAM-Less SSD can work pretty well. However, DRAM SSDs come with many advantages which we are going to discuss later in the article.
Why DRAM is used in SSDs? Its benefits
DRAM is a volatile or temporary memory primarily found working alongside CPUs. As compared to the NAND Flash chips (SSD storage), the DRAM is pretty fast. DRAM is based on capacitor storage while the NAND Flash memory is transistor-based storage. If your SSD has a DDR5 DRAM chip, it can have a speed of around 25,000 MB/s. This speed is utilized on SSD to handle many operations.
1. Data Mapping cache
The first application of DRAM is to store the data mapping tables. This table is also called the Flash Transition Layer. The purpose is to translate the Logic block address to physical locations on the NAND flash. To locate the data, the controller needs to know its location which is stored in this table. It also keeps track of the available free blocks on the flash memory chip. Quick access to this table and its fast working can streamline this process and help in increasing read and write speeds.
With the help of DRAM, the latency of data access is reduced because the controller doesn’t have to wait much to find the data location.
In case the SSD is using HMB (your system memory) as a data mapping cache, it would increase the latency because the communication has to be done through the PCIe/NVMe interface.
2. Read/Write Caching and Buffering
DRAM also works as a write buffer. As a buffer, it is utilized to hold the incoming data temporarily before it is written to the NAND Flash. This helps the controller to organize multiple small write commands in combined larger blocks. This reduces the write amplification because you can combine multiple incoming data streams and store them in a single block without the need to erase multiple blocks. Also, it increases the perceived write speed because NAND Flash memory is very poor when it comes to writing random data.
This write buffer has limited capacity. If your drive has a 1GB DRAM cache assigned for write caching, the write speed will come to the real write speed after this 1GB cache is filled. However, this is the case only with larger random write operations. In sequential write conditions, this cache generally keeps emptying itself by writing the data quickly on the NAND Flash.
As a read cache, the DRAM chips are used to store the frequently used data also called the hot data. This increases the system’s performance because the read latency is reduced. In caching, copies of this hot data are stored to reduce the access time.
3. Help in running algorithms
DRAM helps in garbage collection by keeping track of which data needs to be erased or moved quickly. When combined with the TRIM command, the DRAM can make this process even smoother. It also helps in running wear leveling algorithms by increasing the speed of identifying the blocks that are not used frequently.
Along with this, the DRAM stores the metadata such as error correction codes and bad block information. It helps with over-provisioning and TRIM.
An SSD Without the DRAM
An SSD without its own DRAM will either use some part of the NAND Flash memory or your system’s memory. If it is using the NAND flash memory, it will be used in SLC mode (SLC stores only one bit per cell). Otherwise, HMB (Host memory buffer) is the most common caching mechanism for DRAM-Less SSDs.
An important thing to note here is that the SLC-Cache or Pseudo-SLC Cache is used just for caching and not storing the FTL. While the system RAM in HMB mode can be used for all the things that a real SSD DRAM can do. In case of no DRAM and no HMB, the pseudo-SLC might be used for caching while the FTL is stored on the ordinary NAND flash which will be slower than both DRAM and HMB.
All these things like write caching, algorithms, read caching and table mapping will run even though an SSD has a DRAM or not. It is just that the speed and efficiency of these operations will be reduced.
Because the DRAM is installed on the SSD itself, its latency is reduced. Also, the connection is direct and the SSD is programmed to utilize its own DRAM in an efficient manner.
Drawbacks of a DRAM-Less SSD
Caching is crucial in SSDs. Even without the DRAM, the SSD will use some sort of caching to optimize its working. But, there are drawbacks of not having own DRAM chip.
1. Drawbacks of HMB caching
This feature came into the market when the NVMe 1.2 was launched. With this feature, the SSDs take some part of your system RAM and use it as its own cache. It gave an obvious advantage of better performance as compared to the DRAM-Less SSDs. It reduced the mapping-table access latency by a huge margin.
But, because the communication has to go through the PCIe or SATA bus, this increased the latency and reduced the performance. Also, the system’s RAM capacity is compromised especially on low-end computers. In heavy load scenarios, the interfaces can be full of ongoing processes by other components which can result in degraded SSD performance. If the system has a slow RAM, the SSD has to suffer the circumstances.
2. Drawbacks of Pseudo-SLC Caching
There is no set size of Pseudo-SLC caching. It is a part of your main SSD flash storage set aside and set to work as SLC NAND. It is dynamic in nature and can shrink as the drive starts to fill up. Also, because it is repeatedly written by the incoming data and then erased, this increases the wear and reduce the lifetime of the NAND Flash.
In order to convert the MLC, TLC, or QLC NAND Flash to SLC, the firmware has to perform some complex operations which can lead to bugs and inefficiencies. The biggest drawback is that the pseudo-SLC caching isn’t that fast. It is generally much slower than the HMB caching as well.
3. Drawbacks of Direct-to-TLC/QLC caching
There are some very cheap SSDs that might be using the direct caching method. However, they are available in the market. These drives do not have any dedicated caching mechanism. Instead, the controller uses caching algorithms using the same memory that is used for the storage. These drives perform worse in mixed workloads where the drive has to both read and write at the same time.
There is a lot of wear happening because there is no cache to handle the incoming data and write directly on the NAND Flash. It also increases the write amplification. Especially in random writes, the performance will be very bad as compared to both HMB and pseudo-SLC caching.
Benefits of DRAM-Less SSDs
1. Lower Price
DRAM chips are expensive and if your drive doesn’t have one, it would definitely impact its price. DRAM-Less SSDs are generally cheaper as compared to the DRAM SSDs with the same specifications. So, if SSD price is a significant factor, DRAM-Less SSDs can be very good.
2. Lower Power Consumption
DRAM chips require additional power to work especially because it is a capacitor-based memory. This reduces the overall power consumption of SSDs. The difference isn’t that huge but still, it can be a benefit, especially in laptops and mobile devices.
3. Sufficient performance on low-end systems
No matter how much compromise you have to do when going for a DRAM-Less SSD, the performance is still better than the SATA SSDs, In fact, the performance is mostly sufficient enough for average systems. For moderate workloads and less demanding applications, these drives will deliver good performance.
However, if your work includes heavy read/write operations, you should choose a drive with DRAM. For tasks like video editing, AI, 3D modeling, mining, etc, you should choose a DRAM SSD.
4. Less Heat Generation
DRAM is one of the major contributors of heat in SSDs after the controller and NAND Flash chips. Although this heat is worth its cause still, you have got the advantage of a comparatively cooler SSD. Without a DRAM, the controller design will be simpler.
DRAM vs DRAM-Less SSD: Which one to choose?
See, if you are conscious about the longevity, performance, and reliability of your drive, you should go for the DRAM SSD. It will give you many advantages in both the long and short term. The write performance will be better and your day-to-day tasks will benefit. A DRAM SSD will degrade later as compared to a DRAM-Less SSD.
However, if your system has a light workload, DRAM SSD can work well for you. The price will be lower so you have got another advantage for sure. Otherwise, there is no other serious advantage to going for a DRAM-Less SSD.
| Feature | DRAM SSD | DRAM-less SSD |
|---|---|---|
| Cache Type | Dedicated DRAM cache | Uses Host Memory Buffer (HMB) or SLC cache |
| Performance | Generally better performance | Typically lower performance, especially in random input/output operations |
| Latency | Slower when the SLC cache is full or no cache is used | Higher latency due to HMB or direct NAND access |
| Write Speed | Faster, especially for random writes | Slower when SLC cache is full or no cache is used |
| Read Speed | Faster due to efficient caching | Can be slower depending on caching strategy and workload |
| Endurance | Better endurance due to reduced write amplification and wear leveling | Higher write amplification, impacting the NAND lifespan |
| Reliability | More reliable with good error correction and caching | Less reliable in high-load scenarios due to less effective caching |
DRAM vs DRAM Less SSD for gaming
An SSD with DRAM will benefit a gamer the most. This is because there is generally repetitive data utilized while playing games. The read cache in the DRAM will give a clear advantage over a DRAM-Less SSD. DRAM SSDs will be helpful in quick asset loading and game booting.
DRAM-Less SSD, on the other hand, will be good for budget gamers. If you are good with slightly lower boot times and low performance, you are good to go with a DRAM Less SSD.
Don’t compromise on NAND Flash for DRAM
I would not recommend buying a QLC SSD with DRAM while rejecting a TLC or MLC SSD without DRAM. If your drive has TLC or MLC NAND Flash but uses HMB instead of DRAM, you should definitely buy it over a QLC SSD with a DRAM. Samsung 980 could be a good example here with MLC NAND and no DRAM. But, it is better than any QLC SSD with DRAM in terms of reliability.
I hope this helps!