There is no way to declare a preferred block size for Loopback

[Merovius]

The convenience function nbd.Loopback does not yet allow to pass in a preferred block size. It would make sense to allow that - an *os.File backend for example, could derive that from syscall.Stat_t.Blksize. This should either be part of the Device interface, an extra parameter, or a separate struct with options passed to Loopback.