Data Center Strategies

SSD performance numbers

There’s a lot of hype surrounding Solid-State Disks right now, and if you’ve read tech news recently or my last post, you’ve probably stumbled upon some of the impressive numbers coming from the SSD camp. All in all, it looks like SSDs are faster than Hard Disk Drives(HDD). Great…but how does that affect you? Does this mean you have to replace your HDD? What about the servers for your business, and storage, and, and, and…? Well, obviously, I’ve been asking these questions too, and (lucky for me!) I have a X25-M SSD from Intel to evaluate. Let’s see if it holds up to the hype.

First, though, let’s put things into perspective. Much of the SSD hype stems from the fact that over the past 10 years CPU performance has increased 30 fold; whereas, disk transaction performance has been progressing much more conservatively and deliberately. Look at the SSD innovation from the perspective of the theory of evolution. Way back, organisms swam in the ocean, made progress through the years by adjusting fin sizes and shapes, but didn’t see real change until that one fish decided to turn those fins into legs. Then Bam. New era.

Or, take the Humanoids running around an ancient continent. They made slow and deliberate progress using that opposable thumb to hit each other with sticks (remember the opening scene from 2001: A Space Odyssey?), and then one day there’s an evolutionary leap, and we have fire. Bam. New era.

According to the hype, disk technology is making a leap to a new era. Bam. Solid-State Drives. Bam, IBM announces 1 million IOs per second using Fusion-IOs flash based storage.

Our present hard disks have these rotating parts, which, let’s face it, are so Industrial Revolution. The new SSDs with their new FLASH technology seem cool. But just because something seems good doesn’t mean it’ll last. And just as with species evolution, it’s survival of the fittest. Even some promising candidates meet their end. I mean, what ever happened to the giant, prehistoric Megalodon (it was pretty neat)? The Neanderthals? Or the lead actor from 2001: A Space Odyssey? (Okay, maybe he’s still around, but you get the point.)

So I’ve taken the X25-M SSD that – mind you – is intended for laptops, and gave it a try running Windows Server 2008 with hyper-V to see if it really will improve our lives – or better yet, YOUR business.

Here’s the setup – quad core Intel proc with 1333MHz front side bus and 800MHs DDR2 memory. Now the SSD is only 80GB and I wanted everything to run from it. Not exactly a configuration that I would recommend but it’s the best I can do until I get another SSD (hint…). I loaded the Host OS onto a dedicated 25GB partition then, created another partition for the VMDKs. I created two virtual machines – an instance of Vista and another one of Windows Server 2008. Trying to keep it simple I did not load anything else. So the question I asked myself is – will I notice a difference or will I need to tease the difference out. Short answer – no teasing necessary. Take a look at the chart.

The time to boot a vanilla Windows 2008 server went from 53 seconds to 22 seconds – a greater than 2x improvement – not bad. I achieved similar improvements for the virtual machines running on top of hyper-V. For the host boot time, I measured from when the BIOS was done booting until I saw the login screen. For the virtual machines, I measured from the time I clicked the start button in the hyper-V manager until I saw the login screen.

Emboldened, for you techy types, I ran some down and dirty transactions measurements using IOmeter. In the following graph note the huge differential between the HDD (a Seagate Barracuda SATA 500GB) and the Intel SSD. I ran random IO, with 512, 4k and 32k block sizes. The SSD, worst case, has three times the write transaction rate of the HDD. Best case I measured over sixty times the performance. Again, no teasing necessary. For detailed measurements look at this site and this site.

Now imagine numbers like this in your data center. You’re at work, cup of coffee in hand, looking at your newly installed SSDs, and not only are you marveling at the increased performance, but you notice you can even hear the birds chirping outside, rather than that insistent hum from the old HDDs. (Okay, maybe a bit much, but you get the point.)

According to the numbers, do I think SSDs will help improve our lives? Does it make sense to put SSDs into servers? Yes and Ye--…well, maybe. With the present price structure, the advantage to the SSDs is entirely dependent on the application. For example, data centers that rely on high performance will find the SSDs as a must have. On a base level, SSDs are able to boot more than twice as fast as HDDs by my measure, and when dealing with a large number of servers, this makes a substantial difference. However, for companies that rely on greater amounts of storage for movies, music or other types of large files, SSD might not be the way to go…at the moment. On a transactional performance basis, 10 SSDs can replace hundreds of HDDs, but on a capacity basis the cost per gigabyte does not make the switch cost effective with the present price structure. For SSDs to be used with applications requiring gobs of capacity, we’ll have to keep rubbing sticks together hoping for fire.

Neither SSDs nor HDDs seem to be going the way of the Megalodon anytime soon. For now, the incorporation of both devices might be the optimum solution for business needs as the SSDs can increase performance in certain areas all the while balanced with HDDs for cheaper storage per dollar.

Oh, and one not so small thing: we need storage subsystem providers to start shipping product with SSDs in them!

Overall, the X25-M SSD seems to hold up to the hype. But before giving my final conclusion, I’ll have to report on how quickly World of Warcraft loads…that is, unless I get caught up playing it.

Posted by Gene Ruth

SSD - Storage is just the first wave

Solid state disk (SSD) made from FLASH memory technologies are suddenly all the rage, with big vendors like EMC and SUN giving details on their strategy for using SSD to improve I/O performance in enterprise storage. It's not surprising that storage is the first to benefit from the technology for two reasons:

1. Performance: Conventional disk cannot address the growing gap between storage and overall system performance. You can't just make disks spin faster or the read/write heads move more quickly, physics gets in the way.
2. Ease of use: FLASH-based memory packaged as disk (i.e. using the same 3.5" and 2.5" form factors and interfaces) is pretty easy to integrate with existing storage systems, it doesn't take a rocket science degree.

The immediate impact will be on sales of high performance SAS and FC disk, where performance is prized over capacity. Why buy (and subsequently power and cool) ten 72GB 15K RPM FC drives if one or two SSD drives can give you better performance and still meet your capacity requirements? Yes, I know the technology isn't quite there yet, particularly for write performance, but it will be, and it will decimate the high-performance disk market.

With SSD in disk drive form factors a foregone conclusion (my bet is most enterprises will have it for at least some applications by 2010), the more interesting question is how the technology evolves and it's long term impact on several areas:

• Server form factors: Some of the form factor of a typical server or blade is governed by the number of drives supported. But why have drives at all, FLASH memory doesn't have to be packaged that way, it could installed in slots on a motherboard for example. So FLASH bring about new more dense server form factors as storage performance, power consumption, and storage form factors change.
• File systems: All the file systems in widespread use today are designed to overcome or at least mitigate the inherent limitations of conventional disk. For example many file system optimize placement of disk blocks for a particular file to minimize disk head movements caused by fragmentation, others employ defragmentation tools to address the problem. But FLASH-based storage is truly random access, performance is the same for every single block on the device, rendering the whole concept of fragmentation meaningless.
• I/O interface protocols: Protocols like FC and SAS are designed for use with conventional disk, but they don't make sense for high-performance I/O from memory, which would be better suited to a pure memory type interface without all the storage protocol overhead.
• Main memory: At first glance this one doesn't seem that attractive, after all FLASH is much slower than DDR2 DRAM isn't it? But it doesn't have to be that much slower, as Spansion are demonstrating with their NOR FLASH-based DIMMs that plug right into the memory slots on the motherboard. Read performance is not quite up to DDR2 speeds, but it's close enough, and 32GB/DIMM (that's 256GB in eight slots) is very attractive for applications that primarily read from a database that can now be loaded into memory.
• Operating systems: Changing the performance ratio between storage and compute may open up new possibilities in OS design, for example the paging system could be implemented on FLASH storage interfaced directly to the memory bus.

I could go on (and probably will do in the future :-), but that will do for now.

Posted by: Nik Simpson

The Second Coming of the Hard Drive?

Recently, there has been coverage in the IT press of an announcement of work being done in Holland on a technology called Heat Assisted Magnetic Recording  (HAMR) for disk drives. In short, HAMR involves using a laser to write data on a conventional magnetic disk. Almost without exception, the coverage has focused on a claim that a lab experiment was able to change the magnetic state (i.e. whether it's "1" or "0") of a bit on a magnetic recording media, 100 times faster than conventional hard disk technology works with headlines trumpeting "hard disks get 100 times faster" which sounds great. Of course it's not true!

First, this a lab experiment working with magnetic media with storage densities roughly 1/10th of what we currently have and is 10-15 years away from a production device, so don't hold your breath. More importantly, a hundred fold improvement in write speed for a disk will have no perceptible impact on hard disk performance, for the simple reason that drives today are not limited by the speed with which bits can flipped. Today, a hard drive is capable of around 200 discrete I/O operations a second, that means on average any I/O takes around 5 milliseconds to complete. Nearly all of that time is used moving the disk read/write head to the appropriate location on the hard disk. To see how just insignificant the proposed performance improvement is, lets look at the claim:

The new technology can flip a bit in 4x10^-14 seconds, today, the figure is more like 4x10^-12 seconds

That almost sounds impressive until you look at it in the context of the time to complete the I/O operation of around 5x10^-3 seconds. We are talking about shaving a few billionths of second off an operation that takes several thousandths of a second to complete. So, no need to get the party hats out just yet! At the same time, if perfected the technology would lead 2.5" drives that can hold 40-50 TB of data, about 100x times more dense than today's drives. At the end of the day, only extraordinary improvements in drive mechanics to reduce seek time and rotational latency will have any detectable impact on disk performance. The problem is that, for drive mechanics we run into the dictates of hard physics such as inertia (speeding up head movement) and centripetal forces (spinning the disk faster) that can't be worked around easily.

No amount of improvements areas of drive performance that don't reduce seek & latency penalties will have any significant impact on real world I/O performance. The reality is that hard drives are Dinosaur technology that has to be replaced before we'll make any significant headway in addressing the storage performance gap, and so far the progress on technologies such as holographic storage have been glacial, while flash-based solid state disks making their way into laptops are a long way from being practical in enterprise storage applications because of the longevity, cost, and storage density of the devices.

Meanwhile, if you are looking to increase I/O performance today, the only proven ways are:

• Cache I/O operations in memory (both in the host and the controller)
• Solid state disks for highly I/O intensive applications
• Stripe a logical disk over many physical disks to get parallel read/writes

posted by: Nik Simpson