Flash vs. RAM: Why not choose both?

You won’t find RAM-based SSD (solid state drive) arrays just anywhere. But if you visit an HBA vendor’s lab, you surely will. After all, a single such box can deliver performance on par with a large lineup of conventional storage devices.

Just how fast are RAM-based SSD arrays, and how do they compare with flash-based storage? The RamSan-440 and the RamSan-500, both from TMS, provide an intriguing comparison.

The RamSan-440 is the latest and fastest member of the RamSan family, according to TMS. Released this week, the 4U box can serve 256GB or 512GB of storage in up to 1,024 LUNs. Two 4Gbps FC (Fibre Channel) ports are standard, but the unit can support six more. Or, if you prefer, the box can come equipped with four 10Gbps Infiniband ports.

According to TMS, the RamSan-440 can sustain random reads at 600,000 IOPS and 2KB blocks transfers at a rate of 4.5GBps. At that rate, you could copy the content of a near-full DVD in 1 second. Of course, there is no DVD device capable of keeping up with that pace, but that puts the RamSan-440’s performance into perspective.

By comparison, the RamSan-500 — a heavily cached, flash-based solution that hit the market at the end of 2007 — would take twice as long to complete the same transfer. According to the vendor, the RamSan-500 can sustain only 100,000 IOPS –- a remarkable rate, but one that pales in comparison to what the RamSan-440 can do. Of course, that 6x performance gain comes with a similar hike in price. TMS dodged my questions on pricing, but did suggest $100 per GB for the RamSan-500, a device that you can order with as much as 2TB capacity, while the RamSan-440, at $500 per GB, carries a more challenging sticker price.

How does the RamSan-440 solve the apparent contradiction between volatile RAM and storage persistence? Well, redundant power supply, of course, plus batteries that in case of power failure will keep the array’s memories alive for up to 25 minutes, according to TMS. But there’s more: The RamSan-440 also includes five removable modules with enough flash-based memory to ensure a full RAM backup.

In essence, you get a very fast RAM-based storage backed up to permanent flash storage — an interesting proposition. In basic configurations, the system will automatically initiate a backup to flash in the event of a power failure. The 512GB model can complete this backup in 6 minutes — well before the batteries are completely discharged. To the best of my knowledge, TMS is the only vendor to offer flash backup for RAM-based storage.

It’s an optional feature, but you can also set up the array to do constant background synching of RAM content to flash, which would protect data against double failures in case, for example, batteries misbehave during a power failure.

The five flash modules are protected by a RAID configuration that permits full backup of the RAM content even if one module fails, and can, obviously, be removed. Therefore, should your machine fail, you can easily transfer your backup data to a similar machine.

Did I mention that the RamSan-440’s RAM is also RAID-protected?

But what about power? According to the vendor, the RamSan-440 slurps a paltry 650W at peak. For comparison, I’ve measured the energy absorbed by single 16-drive storage arrays mounting fast disks at around 450W.

Here’s a comparison of a 512GB RamSan-440 unit and a 2TB RamSan-500 model, according to data provided by TMS:

What should catch your eye is that the more expensive RamSan-440 model has much lower price-performance ratios than the RamSan-500. As such, it is a better bargain if speed is a top priority. Of course, the table offers only a rough estimate of those ratios: It will be interesting to see how the two arrays will fare in the SPC (Storage Performance Council) benchmarks.

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