NETGEAR uses BTRFS in their ReadyNAS OS, and implements Tiered Storage in their latest versions. They started with "Metadata" tier only in ReadyNAS v6.9, and then added "Data Tier" in v6.10. The system uses SSDs as Tier-0 to speed up access to the slower HDDs in the system. The description of the system states that metadata will reside on the SSDs in both cases, and that in the "Data Tier" case also newly written data will go to the SSDs first, and then later will get migrated to HDD periodically, or when the SSD tier fills up to a specified level.
ReadyNAS uses BTRFS on top of RAID-ed HDDs in its normal installs - e.g. my system has a RAID5 made of 4 disks, which BTRFS sees/uses as a single device.
Looking at how the Tiering is implemented, it looks like both "Metadata" and "Data Tier" setups are done by adding a second RAID array, made only of the SSDs, to the main HDD RAID array, and transforming the initial single-device BTRFS into a multiple-device one.
What I cannot figure out is how the migration is done, and also how the "Metadata" case manages to separate metadata from data, so that only metadata goes to SSD? Also, how does the "Data Tier" mode direct the writes entirely to the SSD tier?
Any ideas?