In this case we are going to talk about the performance of M.2 SSDs with PCI Express interface for communication, so we will leave aside SATA SSDs that although
How to understand the performance of an SSD
To understand the difference between sequential and random access to an SSD, we are going to make a comparison with real life that everyone should understand. Imagine for a moment that you have two different postmen in charge of delivering the letters.
- We call one of them red and he has his work bag with all the mail he has to send arranged by streets, in such a way that all he has to do is reach a street and deliver the mail door to door and from door to door. orderly way. He will even be able to leave his motorcycle temporarily parked, since he will not need to return to his vehicle to move him. We will call this way of proceeding sequential access.
- On the other hand, the other mailman, whom we will call blue, does not have his bag of letters to distribute in order, he has to continually move around the neighborhood or district on his motorcycle. What will make you deliver less correspondence during your work schedule during your work schedule. So we are dealing with a random access, where a good part of the time is spent looking for the new address.
Unfortunately, the vast majority of computer programs have random access to memory, there is not a single fully sequential program. On the other hand, the one in charge of playing the postman or rather the post office and postman is the central chip of the SSD, which we call the flash controller and it is the one in charge of ordering the correspondence, both sent and received. The postmen would be the memory channels.
Why do some SSDs have less performance than others?
With all this you will have understood that the reason why SSD manufacturers always give the sequential speed as a sample of the performance of their devices is for pure convenience. Since it is the scenario where the best results are given. Now, the chip that acts as the flash controller does not differ much from the memory controller integrated in the CPU to access the RAM, its operation is almost identical, but with the difference that it accesses the flash memory chips in the memory unit. solid state.
The trick from manufacturers is that they tell you the speed at which data is transferred from the SSD to RAM over the PCI Express bus, but they don’t tell you the speed at which the flash controller accesses the drive’s internal memory. Well, if you take for example one that uses a PCIe Gen 4 interface you will see that it reaches 8 GB/s at its maximum point, however, we have units that work at 7 GB/s, others at 6.6 GB/s, others at 5.5 GB/s. The theoretical maximum is not reached due to the fact that it depends on the performance of the SSD controller chip and the type of memory used.
What’s more, because the CPU’s cache is preloaded with the next memory addresses that the system will access, it can tell the flash controller to search the SSD for data in advance. This is where the RAM contained in the drive is important, as it allows the flash controller to sort the information in advance and present it as quickly as possible. Hence, units without RAM, despite being cheaper, perform much worse.
Memory channels also play a role
Going back to the postman analogy, and like RAM, having multiple memory channels means being able to access and search for more data simultaneously. The problem is that with the rise of M.2 drives, with fewer storage chips, we find that high-performance drives increasingly support less simultaneous sending and receiving. On the other hand, although we see chips in a single package, one of the most common tricks is to make each flash memory chip that appears to be actually several different ones.
What do we mean by that? We may have a chip with 1 TB of information that is divided into 4 blocks of 256 GB each in an SSD and, on the other hand, in another we have the same capacity but in 8 blocks of 128 GB. In appearance they have the same capacity, but the second one will perform better since there will not be as much contention in the access to information. So the choice of flash controller and the choice of memory chips influence the performance of the SSD.
Obviously, these are factors that influence performance and manufacturers have agreed to hide this when promoting their units. Hence, the specs and looks look the same, but the performance of them differs from model to model.