A story which may or may not be based on lived experience: one day a Sprite stops booting. JuiceFS is asking object storage for a chunk and getting a 404. We wanted to go fast, so we ran JuiceFS in writeback mode. It records a chunk in its metadata first and uploads the bytes after. If the machine is killed at any time between those two steps, the metadata points at a chunk that doesn’t exist.

So we built a harness that drives fleets of Sprites through write, checkpoint, sleep, wake and restore, over and over, verifying every byte that comes back. We pointed it at 400 JuiceFS Sprites for about 5,000 cycles. A nonzero number of them permanently lost the ability to checkpoint. Well … fuck.

Enter: the Sprites Block Device (SBD). It takes a bucket and presents it to the Linux kernel as a block device, over NBD. The Sprite formats that device as ext4 and mounts it, the same way you’d mount a disk on any Linux box. Duh.

This blog has has a strange vibe like "the last BRAWNDO we made in a bathtub may or may not have killed some customers lmao, try NEW BRAWNDO 2.0 TODAY!" that I didn't realize could work for corporate messaging around an in-house CoW cloud filesystem.