[SSD] Copy and benchmark - please explain.

[akadamy]

Hello
I have a question about copying files inside ssd and atto benchmark results
This is new Sandisk Ultra 3D 250. It has something like DRAM as I understand it. I wonder if everything is ok. Test done on an Acer laptop with i5-6200u.
First, is it normal to copy a large file (over 5GB) to another folder on the same disk at this speed if the disk has DRAM? The speed drops very quickly to a value of 1xx MB /s

What copy speed should then be expected on SSDs without DRAM? Does the disk without DRAM slow down significantly in normal computer use when it is full, e.g. over 80%
The second thing.
I did Atto tests. And the test on a fresh running os (latest win10 pro) looks like this.

I am concerned about the results from 8kb down to 512b. They are probably very low and very unstable-they differ a lot each test. But when boot windows PE win 10 from pendrive results are better. Where does this difference come from. Drivers etc. ok.

Why is there such a clear decline under running windows.
And this is CDI at the end.

 

[Maxx]

DRAM isn't directly related to sequential performance. File transfers are single queue/thread so won't be as fast as Q8T1 for example. Copying on the same disk will be slower, too. Lastly, once you are out of SLC cache (which is only 3GB on that SKU) you are hitting TLC, which at that capacity is at most 250 MB/s. See the 500GB version here - a small SLC cache (>500 MB/s) followed by TLC. Halve that performance for the 250GB SKU. Then lower it further for being single-queue/thread (this is QD32) plus copying on the same drive. Voila, 127 MB/s.

Your worry about results for smaller file I/O sizes is also unnecessary - with less interleaving, speeds will be lower. Smaller files/operations interleave less. When reading 4KiB, for example, the drive still has to pull an entire page (16KiB), so even with fast reads that would limit you to say 50 MB/s, although the higher latency of the AHCI protocol (SATA) can lower this further.

 

[akadamy]

Thank you very much for explanation Maxx Sorry, my English is poor - thank god for google translator
I read somewhere that this drive has 256Mb DRAM but I haven't seen anything about the 3Gb SLC cache. Does this mean that a 3GB file should be copied at maximum speed? Or maybe half of the 3GB for reading and half for writing? Honestly, I expected the files on the disk to be copied faster.

If I buy a disk without DRAM, but with some type of cache, will the similar copying be at min. 120 MB / s?

Your worry about results for smaller file I/O sizes is also unnecessary - with less interleaving, speeds will be lower. Smaller files/operations interleave less.

I am more or less aware of this.
My point was that the RW speeds of 512b, 1, 2, 4, 8kb are not only probably not the best, but there is a difference between the system running from this disk and the test from the system running from a pendrive.
I checked that changing windows power plan to high made the results a little better. But what if someone doesn't want to use this mode?
I don't know if it's a disk defect or a normal thing.
By the way - are these R / W tests for samples of such sizes 512b, 1kb etc.?
I would like to know how such poor and uneven performance in this range (512b -8kb)affects the daily use of programs. I don't know, maybe I should return this drive and buy another one ...
Best regards

 

[Maxx]

DRAM doesn't operate as a typical write or data cache. It's for mapping/addressing metadata (table of contents). SLC is the native TLC/flash in single-bit mode, it's a temporary data/write cache. On drives with static SLC like that one, it's quite small, I estimate 3GB on that capacity. The advantage of this is that its performance is consistent, disadvantage is that larger transfers will hit TLC.

Many DRAM-less drives have a massive SLC cache but this shrinks with drive usage and further is very, very slow outside of SLC. Nevertheless you're still limited to some degree by capacity. There are 500GB drives w/DRAM that will be 500 MB/s across the entire drive, though, but if you're copying from/to the same drive you're looking more at 1TB.

Modern flash has 16KB physical page sizes, your OS has a 4KB cluster size, and then there's 512B sector size. So your file system will emulate 4KB physical sector with the 512B logical size, then the SSD itself has to translate 4KB logical page sizes to 16KB physical page sizes. In order to use more than one flash die at a time the file must be a multiple of that page size, basically. There are other possible limitations for small file sizes, e.g. CPU or chipset, however.

The Ultra 3D is actually a very good drive. The capacity is just far too low with modern 3D TLC.

 

[likeman]

a lot of websites call the dram cache on SSDs when they are not cache it's just dram for page table (so it can locate where the data is actual stored on the SSD quickly) and wear leveling ,, all Writes go more or less directly to the nand the Dram is not used) Genenaly Writes are what are affected on dramless reads not so much, SSDs that have Full power loss protection (enterprise SSDs) Might use the Dram as actual cache but they norm have to be configured to do that

down side with Dramless SSDs is that the page table has to be looked up or writen directly to the nand as it is doing a read and write operation which can slow the disk down once the disk is more then 40-60% full (also Dramless SSDs might have shorter life then a Dram based SSD due to wear leveling)

should note the Sandisk (the modern ones) related WD Blue SSDs that are Classed as Dramless actually have 10-16MB of SRAM inside the controller to mostly mitigate the lack of dedicated Dram (wd Green SSD is totally dramless)

maxx you wont get 1TB/s over sata 600 as total available bandwidth is around 550MB/s per cable (it does not have separate up and down pipes, unlike SAS witch does or can depending on backplane support and controller and disks used)

 

[Maxx]

maxx you wont get 1TB/s over sata 600 as total available bandwidth is around 550MB/s per cable

I know, I said he'd need to get the 1TB SKU in order to ensure he maxxes out a copy to SATA speeds.

 

[akadamy]

Nevertheless you're still limited to some degree by capacity.
[...]
The Ultra 3D is actually a very good drive. The capacity is just far too low with modern 3D TLC.

Hello
Earlier you wrote. Halve that performance for the 250GB SKU
I don't understand why a smaller disk has twice lower performance. It's unfair
If you had to choose - Ultra 3D or MX500 of the same capacity 250GB - what would you choose?
I have high suspicions that you have something to do with this
https://www.reddit.com/r/NewMaxx/wiki/basics

 

[akadamy]

down side with Dramless SSDs is [...] which can slow the disk down once the disk is more then 40-60% full

I don't know anymore what to think about it and how big is the slowdown? Apart from the fact that, for example, files may copy slower. There are so many conflicting opinions on this topic on the internet

 

[drescherjm]

I don't know anymore what to think about it and how big is the slowdown?

I think its normal and expected.

I don't understand why a smaller disk has twice lower performance. It's unfair

This has always been the case. SSDs get their high performance by having multiple NAND chips which basically form an internal RAID0. The smaller drives have less of these chips.

In 2020 I would not recommend purchasing a SSD that is smaller than 5XX GB for a PC device. For a Raspberry Pi these would still be great.

 

[Bowman15]

Like others said that is normal and expected. Your SSD is fine. More GB, equals more chips and more parallel threads which gives you more performance.

That is a layman explanation.

 

[rive22]

If you want very fast same drive copying then a 1-2TB TLC NVMe is pretty much the way to go for the best bang for buck. Otherwise get a second drive to eliminate same drive copying needs.