Triple-Level Cell TLC NAND based SSDs perform usually quite well, but when you copy a large amount of data to the drive without and idle time you’ll often find a large drop in write speed. TLC NAND is great in applications where write operations are limited , but is usually not recommended for critical systems that have heavy write operations as they have lower endurance ratings than SLC or MLC NAND and of course sustained write performance isn’t stellar. In recent years drive manufactures have been figured out that by using SLC or TLC treated as SLC as a cache they can keep the drives overall write performance high as long as the amount of data being written to the drive fits in the cache. If you overflow the cache, you are then writing directly to the TLC NAND and the write performance will drop down to that level. It should be noted that the SLC cache will clear once the drive idles, so this only impacts long writes that are many GB in size. This might not be a typical workload scenario for this ultra-value or mainstream drives, but still something worth pointing out!
Let’s take a look at some TLC and MLC drives to see how they handle sustained write scenarios.
When you average the test results over the 45 second period that we are focusing on these are the average speeds that you come up with:
Not bad, but the performance on the ADATA SU800 512GB SSD does eventually drop off!
The SLC cache for the SU800 is a bit different than most drives as it one third (1/3) of the total available capacity as “SLC” cache. In our testing we found that at 342 seconds into the sustained writes, the performance of the drive went from an average of roughly 521 MB/s down to 80 MB/s on average. This means one would have to write around 174GB of data to reach this point of lower sustained write performance. Roughly 171GB is 1/3rd of the capacity of the 512GB drive, so this numbers is close to what ADATA told us to expect. The lows were down to 40 MB/s, so just like most any TLC drive you’ll get performance drops once the SLC buffer has been filled up and you are writing straight to the 3D TLC NAND Flash. Sustained writes are the weakness of any TLC based SSD, so just be aware of it!
Let’s take a look at some common benchmarks!