Use QLC SSDs as RAIDZ (scientific archive)?

We have implemented the solution. The QLC drives appear to be fine for the use that we make.

However RAIDZ2 showed to be non-practical:

The combination ashift=12 with 16K recordsize (appropriate recordsize for our DB) leads to a high price paid on parity.

Using RAIDZ2 we had two 4K parity blocks written for 16K actual data. One third of the storage was used for parity. We therefore moved back to striped mirrors.

Due to how HP drives report their SMART info, the provided data are not tremendously useful. That said, attribute 173 should be the worst-case erase count (ie: wear) of NAND blocks. With only 26 max erase cycles after 6 months, your SSD should be good for 3000 / 26 / 2 = ~57.7 years.

This is clearly an exaggeration, as much before that you will need to replace something else in your server (or even the SSD itself due to unexpected controller/NAND failure). It is, however, a good starting point to evaluate QLC enterprise SSD: even with 1/10 the endurance, you will be in the ~5 years service time - the same as their warranty typically cover.

Moreover, enterprise QLC drives generally have NAND chips rated at ~1000 cycles, so real-world endurance should be significantly higher than the 5 years reported above.

Coupled with the fact that, as per your question & comments, these SSDs are going to spend most of their time in read-only workload, going with QLC drives should pose no issue at all, unless the slower write speed of QLC drives is of any significance for your workload or you plan to leave your server unpowered for extended periods of time.

Regarding RAIDZ2, it can be a good choice for SSD but be sure to create your pool with ashift=12 and to set a reasonably small recordsize property (I strongly suggest 16K rather than the default 128K value).