Well, array=battery backed controller with a bunch of disks hanging off of it. Actually, there is latency associated with every sync. What I've seen on HDS arrays with Linux boxes and 4GB fibre channell adapters is about 200us per 4KB block write. That is very very good for disk. It's also slower than memory access by many orders of magnitude. This was about 3 years ago. Things are bound to be faster by now, but still not as fast as RAM.

I don't think it's unreasonable to want to write faster than an I/O subsystem can handle. Maybe it's not for every workload, but that doesn't mean it's for no workload.

The distinction I wasn't being clear about was that the storage array (the thing with the RAID controller, cache and disks hanging off of it) is not being saturated if a single transaction log file is being continuously appended to. But that headroom in the array does not translate to more throughput for the transaction log. I don't know if it's an important distinction.