> , an SSD is necessary in order to guarantee something is on stable storage with latency better than spindles can provide
Interestingly enough, if we ignore current hard drive firmware, I don't agree with this. In the context of sending arbitrary random sequences of writes to blocks, sure. But in the context of having a database or filesystem that wants low latency writes? My guess is that you could accomplish this if you track the location of the drive head and spindle. The last time I tried anything like this, though (talking to /dev/sdb, a new 7200 RPM WD Black laptop drive, from userland) I could only get about 1.5 ms, per block write + fsync (iirc -- the numbers 1.3 ms and 2.0 ms ring a bell too). I didn't try writing near the middle of the disk, though, so it could have been drifting the drive head off the track each time for some reason. There's just no hope in general, given current rotationals, when a rotational drive takes ~250 microsecs just to read a 4KB buffer from memory. They just don't care.
If you actually did take advantage of physical information to hold down write latency, garbage collection and keeping startup times low would be a pain (but hey SSDs have gc worries too), there'd definitely be throughput and capacity trade-offs.
Interestingly enough, if we ignore current hard drive firmware, I don't agree with this. In the context of sending arbitrary random sequences of writes to blocks, sure. But in the context of having a database or filesystem that wants low latency writes? My guess is that you could accomplish this if you track the location of the drive head and spindle. The last time I tried anything like this, though (talking to /dev/sdb, a new 7200 RPM WD Black laptop drive, from userland) I could only get about 1.5 ms, per block write + fsync (iirc -- the numbers 1.3 ms and 2.0 ms ring a bell too). I didn't try writing near the middle of the disk, though, so it could have been drifting the drive head off the track each time for some reason. There's just no hope in general, given current rotationals, when a rotational drive takes ~250 microsecs just to read a 4KB buffer from memory. They just don't care.
If you actually did take advantage of physical information to hold down write latency, garbage collection and keeping startup times low would be a pain (but hey SSDs have gc worries too), there'd definitely be throughput and capacity trade-offs.