Well, the thing about reliability is that you can't really guarantee it by testing one particular scenario.
It seems to me that neither the old nor the new version of the code is really "durable" as I would understand the word. The old version made a write syscall per batch, but doesn't say it also did an fsync per batch. The new version writes data to an mmap'ed file, and calls fsync in the background.
So both versions are "durable" in the sense that written data is preserved even if the process gets killed, because it's in the OS page cache. But in both versions, a write can be completed before the data actually makes it to disk, so a power failure will lose acknowledged writes.
It seems to me that neither the old nor the new version of the code is really "durable" as I would understand the word. The old version made a write syscall per batch, but doesn't say it also did an fsync per batch. The new version writes data to an mmap'ed file, and calls fsync in the background.
So both versions are "durable" in the sense that written data is preserved even if the process gets killed, because it's in the OS page cache. But in both versions, a write can be completed before the data actually makes it to disk, so a power failure will lose acknowledged writes.