By Danny Bradbury
PCIe SSD technology is the latest incarnation of the solid-state drive revolution sweeping the data centre. For the most part, SSDs in the data centre have used conventional storage interfaces designed to support mechanical drives, but recently drives have been developed for the high-speed PCIe bus interface. This style of interface is catching on, slowly, so what are the benefits and drawbacks of PCIe SSDs?
PCIe SSD benefits
The biggest benefit of PCIe-based SSD drives is increased performance. With other server-based SSD types, customers were able to forego the mechanical considerations of conventional hard disk drives(HDDs) -- suddenly rpm measurements became irrelevant because there were no moving parts. But with those types of SSD, the SATA-based interface limits the capacity of the bus that transfers data from the SSDs to the processor.
"Some companies are cobbling something together that combines an HBA or RAID controller with a couple of off-the shelf SATA SSDs on the same board," said Jim Handy, director at market research company Objective Analysis. "That works and brings a lot of speed, but it isn't as good as PCIe SSD."
The key to the higher performance of a PCIe SSD lies in the number of channels a vendor is able to run data through. According to Handy, the fastest SATA-based SSD is Intel's 10-channel controller. Meanwhile, Fusion-io's PCIe SSD devices have 25 internal channels, he said, although the company is using collections of smaller chips than the other vendors.
The higher performance of PCIe SSD (Fusion-io's ioXtreme Pro can deliver a throughput of 2.1 Gbps) makes it particularly suitable for buffering and caching applications, with content delivery high on the list of suitable applications. With vendors such as Cisco heavily promoting the use of video for customer applications, there is likely to be a large future demand for PCIe SSD. Other companies, such as LSI, OCZ Technology, Sun and Texas Memory Systems, are also lending credence to the PCIe SSD market with their own products.
PCIe SSD is also a suitable technology for loading databases to significantly increase performance. A few years ago, operating system vendors touted the possibilities of the increased memory addressing afforded by 64-bit computing, but who has the money for vast arrays of online memory? DRAM and flash-based SSDs are the next best option, especially if feeding data to RAM over a fat pipe. But most SSDs suffer from the same constraint, which is that a single bit of memory can only be rewritten a set number of times before it fails. However, SSDs generally include firmware to handle tasks such as error handling, which mitigates that problem.
PCIe SSD drawbacks
PCIe is a multipurpose bus designed to carry all kinds of data to the processor, but its lack of specialisation also makes it difficult to programme. Storage commands are not defined for this interface in the same way they are for interfaces using storage controllers, such as SATA. While vendors are creating controllers that take advantage of the PCIe bus's extra speed, the lack of standard commands for manipulating storage over PCIe will likely introduce interoperability problems.
Eventually, these storage commands are likely to be folded into PCIe, say analysts, but for now, vendors trying to use the interface as a means of storage I/O have to wing it. This leads to the perennial trade-off of storage interfaces: interoperability vs. performance. Just as with the early days of Fibre Channel, vendors are pursuing high performance but may not be able to guarantee that all of their equipment works together.
Although the PCIe bus itself is compliant, vendors have to create their own controllers to interface with that bus. Unlike established storage controller standards such as SATA, there are no standard storage commands for controllers interfacing with PCIe buses, making it difficult to ensure that one vendor's controller working over a PCIe bus will work with another vendor's controller, even if that too is operating over a PCIe bus.
Because there is no standard set of disk controls implemented in PCIe, standard disk features such as booting from the media and running operating systems on it are more difficult for vendors to implement in a standard way.
"What's been happening with SSDs is that they're emulating disks, but when you directly connect it to the PCIe bus, you don't have to go through the disk controllers, which saves latency and increases performance," said Hamish Macarthur, managing director of storage market research company Macarthur Stroud. But, he said, that makes it difficult to use the disk controllers to manage storage functions such as booting from the drive. "They are developing the protocols to boot, however," he said. OCZ Technology, for example, announced a proprietary PCIe-based SSD late last year with the ability to boot its host machine.
The final problem for PCIe appears to be its unknown track record. Hard drives may be power-hungry and slow, but they are a tried and tested technology. Conventional SSDs use a combination of well-established technologies -- DRAM and SATA or HBA interfaces -- but even these products are still getting off the ground. Running high-performance storage over a bus that wasn't specifically designed for it has a lot of people worried, said Objective Analysis's Handy.
PCIe SSD adoption and market prospects
What is the pace of PCIe SSD adoption? Well, the technology is available, and it delivers tangible performance benefits, but a lack of interoperability stemming from an immature standardisation process appears to be at least one factor hindering its adoption. And market confusion has been exacerbated by the varying prices of SSDs. Faster DRAM-based SSDs will generally cost more than non-volatile NAND Flash-based SSD.
According to Handy, current demand is modest, and PCIe is still very much on the early adopter curve. He points out that Fusion-io, which seems to be leading the crusade for PCIe SSD, sold just 5,000 units last year.
Even though PCIe SSD is experiencing growing pains, it is nevertheless likely to grow. Jeffrey Janukowicz, research manager for solid-state drives and HDD components at IDC, expects a 100% compound annual growth rate in PCIe SSD revenue to 2013.
So, we should expect more companies to enter the PCIe marketplace, and as more established firms jump on board, the changeover from conventional SSD drives running over non-PCIe interfaces will be relatively rapid. This is one bus that enterprise customers will be happy to ride.