I had bcache + btrfs (RAID1) before this but it was a huge waste of space because bcache had to cache two identical copies of the data in order to be effective (since BTRFS and bcache don’t communicate and BTRFS picks from a random disk); that’s half as much cache.

With Bcachefs everything is integrated so it knows to cache only one copy in RAID1 (and it doesn’t even need to hold two HDD copies, the fast/“cached” copy counts). Data is read from the fastest source and every resource is best utilized.

It has RAID modes and it intelligently rearranges data s.t. commonly used files are stored in a fast drive and fetched from there, whereas BTRFS will write to and read from a “random” drive regardless of its speed.

The previous solution of using btrfs raid1 + bcache (not the FS) separately was very wasteful because the cache had to store both/all copies of the data since btrfs picks a random drive to read from.