On Wed, 25 Mar 2009, Linus Torvalds wrote:
Even a suck-ass laptop drive can write streaming data fast enough that people don't care. The problem is invariably that writes from different sources (much of it being metadata) interact and cause seeking.
Actually, not just writes.
The IO priority thing is almost certainly that _reads_ (which get higher priority by default due to being synchronous) get interspersed with the writes, and then even if you _could_ be having streaming writes, what you actually end up with is lots of seeking.
Again, good SSD's don't care. Disks do. It doesn't matter if you have a FC disk array that can eat 300MB/s when streaming - once you start seeking, that 300MB/s goes down like a rock. Battery-protected write caches will help - but not a whole lot when streaming more data than they have RAM. Basic queuing theory.
Linus