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Comment by dragontamer | Hacker News Reader

Except we can already achieve that kind of IOPS increase: by simply using two hard drives in parallel (be it RAID0, or even RAID1 if your driver is willing to split the reads between hard drives).

A multi-actuator drive isn't really "one hard drive" anymore, its really just two hard drives ganged together. While more physically convenient, it doesn't seem to really offer the true 2x increase we're looking for.

Actuator#1 cannot give more IOPS over the data that Actuator#1 is assigned over. You only get more IOPS if you can split the work between the two actuators. Same problem as RAID0 or RAID1 multi-read hard drives (you gotta figure out a way to "split the work" to get RAID0 truly 2x the IOPS).

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RAID0 can't give you a true 2x increase, because reads and writes are constrained to a particular device, and big reads tend to require both drives working together.

RAID1 can give you a 2x increase in reads, but suffers even more than RAID0 when it comes to writes.

Dual actuators, implemented in a straightforward way, can both access the entire drive surface which means they can give you a true 2x increase. Sometimes even better than 2x, because each arm can focus on one side of the disk. For read/write workloads it completely outclasses RAID.

> because reads and writes are constrained to a particular device

That constraint means nothing here. You can issue two parallel reads to two drives in RAID-0 just as easily in RAID-1. The only case where this doesn't work is where you're reading more than 2x the interleave size and you're issuing separate requests for each interleaved chunk. With command queuing, a smart storage system should even recognize the pattern and buffer to reduce the damage, but you'll still pay a cost in extra interrupts and request handling though so it's better to learn about scatter/gather lists.

> they can give you a true 2x increase

I already explained why this isn't actually the case, and have observed it not to be the case with multiple generations of dual-actuator drives. Stop presenting theories based on misconceptions of how disks and storage stacks work as though they were fact.

> You can issue two parallel reads to two drives in RAID-0 just as easily in RAID-1.

Under RAID 0, the odds are 50% that two independent reads are on the same drive. It's impossible to get a speed advantage in that case.

> I already explained why this isn't actually the case

You said they "improve parallelism, not media transfer rate or latency", and I'm arguing about parallelism. Plus large transfers can be rearranged into parallelism (fact, not theory).

And you said that they can face internal contention "elsewhere" but implied that could be fixed.

So that doesn't sound like what you said disagrees with what I said.

> Under RAID 0, the odds are 50% that two independent reads are on the same drive.

If you have a single sequential stream, then no. You'll either have parallel reads across the two drives, or you'll have alternating reads that the aforementioned semi-smart storage system can turn into parallel reads with buffering. If you have multiple sequential streams, then it's practically going to be like random access, which you already put out of scope. So there's no relevant case where RAID-0 is worse than RAID-1 for reads.

But you know what will be worse? Dual actuator drives. Why? Because of what dragontamer (who was right) mentioned, which you overlooked: the two actuators serve disjoint sets of blocks. They even present as separate SAS LUNs[1] just like separate disks would, so you would literally still need RAID on top to make them look like one device to most of the OS and above. But here's the kicker: they still share some resources that are subject to contention - most notably the external interface. Truly separate drives duplicate those resources, enabling both better performance and better fault isolation. Doubled performance is an absolute best case which is never achieved in practice, and I say that because I've seen it. If Seagate could cite something more realistic than IOMeter they would have, but they can't because the results weren't that good.

The only way dual actuators can really compete with separate drives is to duplicate all of the resources that change behavior based on the request stream - interfaces, controllers, etc. Basically everything but the spindle motor and some environmentals, as I already suggested now two days ago. You'd give up fault isolation, but at least you'd get the same performance. That's not what Seagate is offering, though.

[1] https://www.seagate.com/files/www-content/solutions/mach-2-m...

That sentence is specifically not about sequential reads.

Since you added a huge amount since I replied, I'll make a separate reply.

> But you know what will be worse? Dual actuator drives. Why? Because of what dragontamer (who was right) mentioned, which you overlooked: the two actuators serve disjoint sets of blocks.

They don't have to do that.

I was talking about what you can do with dual actuators, not product lines that already exist.

I didn't realize how mach.2 was designed, though. That's a shame.

> But here's the kicker: they still share some resources that are subject to contention - most notably the external interface.

Each head, even at peak transfer rate, uses less than half the bandwidth of the external interface.

So even if both of them are hitting peak rates at the same time, and the drive alternates transfers between them, things are fine. For example, let's say 128KB chunks, alternating back and forth. Those take .2 milliseconds to transfer. That makes basically no difference on a hard drive.

> Doubled performance is an absolute best case which is never achieved in practice, and I say that because I've seen it.

I completely believe you, about drives where each arm can only access half the data.

> The only way dual actuators can really compete with separate drives is to duplicate all of the resources that change behavior based on the request stream - interfaces, controllers, etc.

Or upgrade them to 1200Mbps, which isn't a very hard thing to do.

> I was talking about what you can do with dual actuators, not product lines that already exist.

Since you didn't know they're different until a moment ago, you were talking about both. Don't gaslight.

> Each head, even at peak transfer rate, uses less than half the bandwidth of the external interface.

So two will come damn close ... today. With an expectation that internal transfer rates will increase faster than standards-bound external rates. And the fact that no interface ever meets its nominal bps for a million reasons. Requests have overhead, interface chips have their own limits, signal-quality issues cause losses and retries (or step down down lower rates), etc. Lastly, request streams are never perfectly balanced except for trivial (mostly synthetic-benchmark) cases, and the drive can't do better than the request stream allows. There are so many potential bottlenecks here that any given use case is sure to hit one ... as actually seems to be the case empirically. Your theory remains theory, but facts remain facts.

SSDs achieve their speed in part by combining multiple independent NAND channels under a single controller - each channel is more or less equivalent to an actuator. Their speed vary greatly based on workload parallelism, yet it's still very much one drive.

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