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 performance. From what we can tell the WD Blue 1TB SSD has 8GB of it’s available capacity being used as “SLC” cache. In our testing we found that at 15 seconds into the sustained writes, the performance of the drive went from an average of roughly 519 MB/s down to 307 MB/s on average. This means one would have to write around 8GB of data to reach this point of lower sustained write performance. 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 TLC NAND Flash. Sustained writes are the weakness of any TLC based SSD, so just be aware of it!
Our sources have informed us that for ever 250GB of space that 2GB of SLC cache is available on the drive. That means the 250GB model drive would have 2GB of SLC cache and the 500GB model would have 4GB of SLC cache, respectively. With lower cache that means the drives will see this performance drop quicker, so there is another reason to buy a larger SSD if at all possible.
Let’s take a look at some common benchmarks!