special vdevs were designed for more efficient storage of small files, and of the tails of large files, in combination with dRAID, which is especially bad at dealing with small blocks. So it’s mirrors only because raidz is also bad at storing small blocks, though not as horribly bad as dRAID.
But conflating tails as well as small files, makes svdev somewhat less desirable with raidz, and harder to plan for. Metadata-only svdev still makes sense with raidz, but then it only requires small capacity, and is only useful if there are many (re)writes; for read workload, tuning L2ARC for metadata provides similar benfits without introducing a pool-critical vdev.
8 TB SSD are huge compared with a 5-wide raidz2 of 30 TB drives. In theory, you could have one or two 3-wide mirrors of 8 TB as svdev alongside the 5*30 TB raidz2 (to keep a consistent redundancy), but does that make sense?
