Yeah, I found the seek issue too. Fixed it.
We did a lot of testing with this change on various devices: rotational HDD, slower SSD, and faster SATA SSD. We see disk utilization is pretty good with these changes.
The commit is now merged. Try it out!
https://github.com/dgraph-io/badger/commit/bb1d12feefcaa7102ead58a53a91e4ae47ae5453