As of ReadyNAS OS 6.9, you can add metadata tiers to Flex-RAID volumes.
Metadata tiers are special RAID groups that offload btrfs metadata from your slower spindle hard drive RAID groups to a faster SSD RAID group. Metadata tiers provide faster writes, which is beneficial for volumes that write many small files (8KB or less), or files that are re-written periodically (like backup images, virtual machines, or databases).
Most metadata tiers are recommended to have two or more SSDs. In RAID 0/JBOD volumes, the minimum number of SSDs is one. Any metadata that exceeds the total size of the tier will fall back to writing to the spindle hard drives. To avoid this, either expand your existing tier to contain more drives, or add additional tiers.
Before adding a metadata tier to your volume, review the Volumes screen for your current metadata size. Minimally, you should exceed the existing size of your current metadata usage and planning for continued metadata accumulation going forward. Two SSDs in a tier will act like a RAID 1. After expanding from a two-SSD tier to a three-SSD tier, your tier will act like a RAID 5. You can convert your metadata tier from RAID 5 to RAID 6 after three disks by adding parity.
On volumes with multiple metadata tiers, metadata is written first to the tier with the most capacity available, if the tiers have differing available capacity levels. Once multiple tiers share the same available space, metadata will alternate between all available metadata tiers.
It is not recommended to mix different disk types in the same RAID group or tier.