I don't understand why so much emphasis is put on optimizing for sequential reads and writes. Sequential IO is not something that happens in the real world except in rare circumstances. In most applications IO is random, even if it's a log file.
I am all ears... how are log writes random?
The twain are similar, but aren't exactly the same. Optimize for the cases where you are writing multiple blocks in a chunk, with contiguous free block allocation policies and write behind and whatnot, and you haven't in reality quite optimized for the case where what you're actually doing is overwriting the tail block in the file repeatedly, oftentimes changing as few as mere 10s of bytes in that block with each read-modify-write request.
(And yes, logging mechanisms often do like to ensure that logs are synchronously flushed to disc line by line, effectively and intentionally defeating write caching and write behind optimizations.)
> At base, most logfile output is a read-modify-write
It's worse, actually, as you never can tell if this would be RMW to the same block or to the other free block. In the former you waste the writes for the whole blocks. In the latter you can hit a write amplification despite never have been writing any meaningful amount of data.