is this the DRAM on SSD that they been talking about?

[foxbill86]

is this the DRAM on SSD that they been talking about? I am not a storage type of guy. Correct me if I'm wrong but the LEXAR NM790 12nm Controller is the DRAM right? compared to NM710 with no controler/DRAM?

 

 

 

[Needfuldoer]

Every SSD has a controller. That's the part that reads and writes the data to the NAND flash.

 

Some SSDs also have DRAM connected to their controllers that acts as a buffer, since DRAM is faster than NAND.

 

Some SSDs use a chunk of faster NAND flash as their read/write buffer, and some reserve a chunk of system memory as their "DRAM cache".

 

According to the product page, the Lexar nm790 uses "Host Memory Buffer" (system RAM) and a chunk of faster SLC NAND flash as its cache.

https://www.lexar.com/product/lexar-nm790-m-2-2280-pcie-gen-4x4-nvme-ssd/

 

Quote

Host Memory Buffer (HMB) 3.0 enables the drive to utilize DRAM memory on your device, and Dynamic SLC cache writes to single-level-cell memory to maintain speed for smooth and seamless performance.

 

Listing "compact and slim" as a feature of the nm710 is just marketing fluff. Of course it's "compact and slim", that's its form factor! All 2280 SSDs are "compact and slim"!

 

[Somerandomtechyboi]

5 minutes ago, Needfuldoer said:

According to the product page, the Lexar nm790 uses "Host Memory Buffer" (system RAM) and a chunk of faster SLC NAND flash as its cache.

https://www.lexar.com/product/lexar-nm790-m-2-2280-pcie-gen-4x4-nvme-ssd/

Basically the nm790 is a high end dramless gen4

 

Get an m482, xs70, s70 blade, gm7000, 250s/h, adata premium, etc. If you want a high end dram gen4

[mariushm]

The DRAM is basically one of the tiny chips you find on a regular DDR4 or DDR3 stick :

 

 

Here you go, 8 DRAM chips  ...  maybe another 8 on the other side.  SSDs may use slightly different dram chips (ex DDR4L) but the basics are the same.

 

The SSD controller can use one or more of DRAM chips to store temporary data into that memory, because reading data from these DRAM chips is faster compared to reading the data from the flash memory chips.

 

The downside is that making SSD controller use DRAM means a larger chip, because the controller chip now has to include an extra  memory controller and the chip must only include the contacts for all the wires going to the DRAM chip or chips - that's why on cheaper SSDs, they use DRAMless SSD controllers, because those cost less money and they can have bigger profits by using such controllers.

 

Here's an example of a nvme SSD with a DRAM chip, the WD SN850  :

 

From left to right, the SanDisk chip is the controller, the rectangular chip is the DDR4 memory, the big two chips to the right are the flash memory.

The small chips are voltage regulators (converting 3.3v to 2.5v or 1.8v or other voltages the chips need) and maybe firmware chips (like the bios chip), if the ssd controller doesn't have built in memory for it.

 

 

 

 

DRAM is not generally used to cache file writes, having DRAM doesn't mean the drive will write faster to the SSD.  Most drives will fill the DRAM with a lookup table, to quickly find where in the flash memory chips some piece of data is located (because data is spread all over the flash memory chips, unlike with mechanical drives where data is kept in concentric tracks, ordered sequentially. 

[foxbill86]

11 minutes ago, mariushm said:

The DRAM is basically one of the tiny chips you find on a regular DDR4 or DDR3 stick :

 

 

Here you go, 8 DRAM chips  ...  maybe another 8 on the other side.  SSDs may use slightly different dram chips (ex DDR4L) but the basics are the same.

 

The SSD controller can use one or more of DRAM chips to store temporary data into that memory, because reading data from these DRAM chips is faster compared to reading the data from the flash memory chips.

 

The downside is that making SSD controller use DRAM means a larger chip, because the controller chip now has to include an extra  memory controller and the chip must only include the contacts for all the wires going to the DRAM chip or chips - that's why on cheaper SSDs, they use DRAMless SSD controllers, because those cost less money and they can have bigger profits by using such controllers.

 

Here's an example of a nvme SSD with a DRAM chip, the WD SN850  :

 

From left to right, the SanDisk chip is the controller, the rectangular chip is the DDR4 memory, the big two chips to the right are the flash memory.

The small chips are voltage regulators (converting 3.3v to 2.5v or 1.8v or other voltages the chips need) and maybe firmware chips (like the bios chip), if the ssd controller doesn't have built in memory for it.

 

 

 

 

DRAM is not generally used to cache file writes, having DRAM doesn't mean the drive will write faster to the SSD.  Most drives will fill the DRAM with a lookup table, to quickly find where in the flash memory chips some piece of data is located (because data is spread all over the flash memory chips, unlike with mechanical drives where data is kept in concentric tracks, ordered sequentially. 

I bought the Lexar NM710 1TB. So upgrading to a DRAM M.2 is not worth it because "having DRAM doesn't mean the drive will write faster to the SSD"

[GuiltySpark_]

Since the inclusion of HMB, for 90% of customers I've not yet seen an application where paying extra for a drive with a dedicated DRAM cache has been necessary or beneficial. This is great news as you can save some money and still get blistering performance.

 

That was NOT the case back in the day with SATA SSD's. You ALWAYS wanted the better models with the DRAM cache. 

 

Unfortunately, you still have people using old information suggesting that even the good DRAM-less drives today are a problem or bad and they're simply mis-informed for a home user/gamer. 

[mariushm]

How fast a SSD controller writes depends on how many "channels" it has and what kind of flash memory chips it's combined with.

 

Flash memory channels are made out of layers of memory, up to 96-128 even 176 layers of memory, and these layers are grouped into "channels" that a SSD controller can access independently.

When the SSD controller receives a file to write, it can split the file virtually in multiple chunks and send each chunk to one channel, and that's how you get fast speeds.

 

Here's for example the Samsung 990 Pro SSD specifications : https://www.techpowerup.com/ssd-specs/samsung-990-pro-2-tb.d862

 

and for comparison here's a second drive, Crucial p3 plus , https://www.techpowerup.com/ssd-specs/crucial-p3-plus-2-tb.d867

 

 

There's a controller with 8 channels, there's 2 Flash memory chips, and each Flash memory chip is made out of 16 dies each 512 Gbit (64 GB) in size, and each 64 GB die is arranged as 2 decks, 4 planes ...  The controller can read data from an individual die at 1600 MB/s, but in TLC mode, it can write data at up to around 184 MB/s in each die.  So it achieves fast write speeds by writing in parallel, to multiple dies at the same time ...  in practice the controller can't write to all 32 dies at same time to achieve 2 chips x 16 dies x 184 MB/s = 5900MB/s, if it was to write entirely to TLC memory you'd get around 1800-2000 MB/s sustained.

 

Crucial drive has a weaker controller with only 4 channels, and is made with QLC which is much slower, it can read from a die at up to around 700 MB/s but it can only write to a die at 27 MB/s so without a pseudo-SLC cache, the drive would write at 100-200 MB/s to the flash memory chips. In pseudo-SLC write cache, it can write at up to 4.2 GB/s until the write cache is full  - by design the whole empty space can be converted to SLC mode to get faster speed (if you have 100 GB of free space, it's like having 25 GB of slc memory... if you write less than 25 GB, you'll write it fast)

 

The Samsung controller also achieves the maximum of 6900 MB/s writes by converting TLC memory to pseudo-SLC where each cell stores 1 bit per cell (see paragraphs below) - this SSD can convert up to around 700 GB of free TLC memory into 226 GB of pseudo-SLC memory and as long as you don't write as much data as the write cache, you'll get close to 7 GB/s speeds. 

 

If you fill up the drive, the amount of write cache will be much smaller.  For example, if you have only 100 GB of free space, that 100 GB is converted to around 30 GB of pseudo-SLC cache and you'll have around 40 GB of write cache (10 GB is permanently write cache). 

If you copy a 50 GB file,  you'll copy around 35 GB at nearly 6-7 GB per second and then the controller has to take blocks of this pseudo-SLC memory and convert them back to TLC memory ... so it takes 24 MB of pseudo-SLC, writes them to a 64 MB block of TLC, converts the 24 MB back to 64 MB of TLC , repeat until it no longer needs to make room ... during this conversion process if you keep writing to disk, the speeds slows down, in the case of this Samsung drive, down to around 1400 MB/s .

 

DRAM helps a tiny bit in write speeds, but only in the sense that the SSD controller can find much faster empty locations in the ram chips where to put the data, if the SLC write cache is full.

 

Most modern SSD controllers take a portion of the empty Flash memory and switch it to a mode called pseudo-SLC mode, in which the flash memory can be written very fast, but can retain less data.

For example, they take a 64 MB block of TLC memory, where each cell can hold 3 bits of data, or QLC memory where each cell can hold 4 bits of data,  and store only 1 bit of data in each cell. So that unused 64 MB of TLC/QLC memory becomes around 20-24 MB of pseudo-SLC write cache which is much much faster to write to.

 

When you're writing a file to the SSD, the SSD controller writes the file wuickly in this hidden pseudo-SLC write cache and when the cache is full or when you're not writing anything to the SSD, the SSD controller starts looking for empty TLC/QLC areas where to store that file in a more permanent way.

 

 

 

[kokosnh]

On 4/13/2024 at 3:44 PM, mariushm said:

Most modern SSD controllers take a portion of the empty Flash memory and switch it to a mode called pseudo-SLC mode, in which the flash memory can be written very fast, but can retain less data.

they take whole CE of a channel for pseudSLC mode, and well that's logical, as you have to write it all in that mode 

as for the DRAM.
SSD uses it to cache a FTL (table with all of the DATA in NAND), and it cache all of it, in it. 
HBM usualy only takes 32 to 64MB of host RAM, and cache, only small fragment of the FTL tabble, so it can map only around 32-64 GB of NAND (it helps with benchmarks, but reading older data is hit with latency penalty, as SSD have to read the FTL from NAND, instead of faster cached one in DRAM, or HMB). 
DRAM also helps with sustained writes, as FTL is constantly rewriting and needs to be flashed to NAND all the time, on the DRAMLESS SSD. 

So DRAM is mostly, for low latency, and consistent write performance (assuming fast NAND and controller), is it worth paying more, depends on the price difference.

You usually don't see the difference, but then again, usually you don't see the difference between SATA ssd, and M.2 NVMe SSD.