[forza_user]

The allocator hints only makes sense with at least two devices. Btrfs is metadata heavy, so offloading metadata to faster devices can make a big performance difference. For example a HDD combined with a small SSD or NVMe disk, you would set "data preferred" on the HDD and "metadata preferred" on the SSD/NVMe disk.

One advantage with these patches compared to using dm-cache, lvmcache or bcache is that everything is native Btrfs and fully compatible with non-patched kernels (though the 'hints' won't apply for new writes). Btrfs allows for adding additional devices to existing filesystems, so no mkfs and data-shuffling is needed. If you do not like the result, simply set default hint (type 0). You can then "btrfs device remove" your previously added SSD/NVMe devices.