With the full application of PCIe 4.0 technology and the imminent arrival of PCIe 5.0 technology, the current SSD publicity specifications are becoming more and more amazing, the nominal read speed of many products has reached more than 7000MB/s, and the nominal write speed can mostly reach more than 5000MB/s, it can be said that the performance of SSD seems to be close to the early memory performance. At the same time, many SSDs are also marked with a warranty time of up to 5 years, giving people the feeling that the performance and durability of SSDs have both been greatly improved, so is the truth of the matter?
A read speed of 7000MB/s is difficult to achieve
Write performance is more "water-injected"
The first thing that needs to be clear to everyone is that the so-called 7000MB/s or higher reading speed refers to the sequential reading speed of the SSD, what is the sequential reading speed? Generally speaking, sequential reading means that the sector address of the SSD read this time and the sector address at the end of the previous read are completely adjacent and not spaced. For example, after the SSD completes the first read, the next data that needs to be read is next to the sector where the data is first stored. Simply put, all the data that needs to be accessed is arranged in order, and SSDs can be very convenient and efficient to "go through the door in turn". In real-world applications, such sequential data files that can be accessed efficiently by SSDs are mainly single large-capacity files, such as video, ISO files, and compressed packages.
However, in real applications, there is also a very common read and write behavior - random read and write, SSD after completing the first access, the next sector that needs to be accessed may be spaced a few sectors apart, and then the next sector that needs to be accessed may be spaced a few sectors, obviously such access efficiency is far less than sequential read and write, so the random read and write speed of SSD is also much slower than sequential read and write. At the same time, in real life, SSDs also need to carry out a lot of random access - such as running games, programs, operating systems composed of various small files will involve reading and writing to various internal files. In fact, when simply transferring the League of Legends game installation files made up of more than 3,000 small files, you'll also find that the read and write speeds of the SSD are much lower than when transferring video or ISO files.
So the SSD is nominally 7000MB/s or higher, which simply refers to its speed when reading sequential data files. In fact, in practical applications, even if it is difficult to transfer large-capacity ISO files and video files, it is difficult to achieve this indicator. Because this 7000MB/s indicator is measured by benchmark software such as CrystalDiskMark, this kind of software does not really test the speed of the SSD when transferred to and from other storage devices, but generates a test file on the SSD to be tested, and the reading and writing speed is measured by the CPU and memory directly reading and writing this test file, so the score measured by the benchmark software is an idealized value. In the real SSD data transfer, to experience a sequential read speed of more than 7000MB/s, it means that you must match an SSD with a write speed of 7000MB/s, but there are very few SSDs with write performance that can reach this standard.
At present, the nominal sequential write speed of most PCIe 4.0 SSDs is about 5000MB/s ~ 7000MB/s, but this indicator may also be suspected of exaggeration. Because many SSDs use SLC Cache caching technology, this technology will use part of the SSD's TLC or QLC flash memory and simulate it as SLC flash memory to improve the writing experience. Generally, when sequentially writes within the SSD's SLC cache capacity, the nominal speed can be achieved, but the manufacturer will generally "forget" the speed at which the SLC cache is exhausted and the data needs to be written directly to the flash memory. The slow speed of SSDs varies according to the type of flash memory and technical specifications. The SSD slow external sequential write speed with better performance can also be maintained above 1000MB/s, such as the use of QLC flash memory, then the SSD's cache-out sequential write speed may even drop sharply to less than 100MB/s, which is not as good as the mechanical hard disk.
The sequential write speeds that vendors annotate for SSDs are also unreliable, and write speeds tend to drop significantly after the SLC Cache is exhausted.
Exaggerated random 4K IOPS you may not experience
Many manufacturers also understand that users are increasingly aware of the importance of SSD random performance in practical applications, so they have offered a pompous publicity: very exaggerated random 4K IOPS performance. Often the nominal random 4K read and write IOPS of the SSD has reached hundreds of thousands of IOPS, and the nominal random 4K read performance of the SSD in the following figure test reaches 650,000 IOPS, that is to say, the random read speed per second can exceed 2500MB/s, is it so amazing? In fact, the manufacturer's nominal random 4K IOPS on the SSD refers to the random 4K performance in a multithreaded environment with high queue depth. That is to say, in the environment where multiple threads are concurrent, multiple random read and write tasks are performed at the same time, and each task has a large amount of data queued for processing to achieve this value, which generally relies on the performance of SSDs on various professional servers. However, for ordinary personal consumer applications, a user obviously does not issue a large number of I/O requests at a time, so in consumer applications, it is actually the most dependent on the SSD's single-threaded, queue depth 1 4K random performance.
The stochastic 4K performance indicator advertised by SSD is actually derived from the results of the third column test project in CrystalDiskMark, and the test results of the fourth column of stochastic 4KQ1T1 are closely related to consumer applications.
As shown in the figure above, the SSD random read and write indicators often marked by manufacturers come from the results of the third column test project in The TotalDiskMark, that is, the random 4K performance at a queue depth of 32,16 threads, but the most influential on consumer applications is the fourth column test project results, that is, the test results at a queue depth of 1 and single thread. In this project test, the specific performance of the SSD with poor random performance is that the random 4KQ1T1 read score is generally within 10,000 IOPS, and the SSD score with better performance can reach more than 15,000 IOPS, but due to the technical limitations of the NAND flash memory itself, the highest test score is currently difficult to exceed 30,000 IOPS. Only high-speed storage devices with new storage media can achieve more than 50,000 IOPS in this score, such as Intel's Optane SSD, but the disadvantage of this product is that it is expensive and difficult for the average person to accept. From the perspective of test results and experience, the random 4KQ1T1 SSD with strong performance can obtain faster speed in game startup and program read and write task execution, in order to bring users a real experience.
There is no need to pay too much attention to warranty time
At present, the slightly more expensive and highly positioned SSDs will mostly advertise its warranty time, and some products will highlight its 5-year warranty time on the poster. We believe that extending the warranty time has some significance, especially for those users who do not have many writing tasks, but for users who often write data on the SSD, they do not need to pay too much attention to the SSD warranty time advertised by the manufacturer. Because most of the built-in SSDs on the market (Note: SSDs installed in the chassis, some products in mobile SSDs will use only time-based warranty standards) will use warranty time + TBW (Total Bytes Written, writable capacity) two sets of warranty standards.
Now the warranty of SSDs is like the mileage calculation and time calculation of car maintenance, and the two standards of TBW and warranty time are implemented at the same time, which is the first-come. That is to say, if your SSD warranty time is 5 years, TBW is 360TB, but after the second year of use, the amount of data written to the SSD has reached 360TB, although there are still 3 years to go before the warranty is valid, but your SSD is also insured and can no longer enjoy free after-sales service. Therefore, no matter how long the manufacturer advertises the warranty time of its SSD, we still have to be patient to point out the specification table and look at the TBW indicator of this SSD. TbW this value is the larger the better, in general, the mainstream 1TB TLC SSD TBW writable capacity is about 600TB, while the 1TB QLC SSD TBW is about 300TB.
Reject pompous propaganda
Read more professional reviews to avoid
Because SSD is still in a Red Sea market, there are currently many manufacturers involved, and fierce competition has forced many manufacturers to exhaust various means to win the favor of consumers, including the above three typical and common exaggerated propaganda. So how to avoid the misleading caused by exaggerated publicity, how to buy good products? Obviously, before purchasing, it is difficult for consumers to understand the real slow write speed of SSD, the real random 4KQ1T1 performance, and the game startup speed from the product introduction specifications, which are the main points that manufacturers will not publicize. Therefore, if you want to understand the real performance of the SSD, there is no other way, or only to look at the evaluation produced by professional media, because most of these indicators are mandatory items for the media.