The considerable reduction in flash memory prices has helped solid state drives reach the mass market and disrupted the long-established trends and traditions of top-performance storage systems. There used to be but one way to achieve high disk subsystem performance which involved building RAID arrays out of 15,000RPM hard disk drives. The more HDDs you had, the higher the resulting performance of the RAID. Thus, a high-performance disk subsystem would always be large and hot.
Then came multichannel solid state drives. Flash memory endowed them with a fantastic read response time while the multichannel design ensured high sequential speeds. They even boasted high write speed thanks to the introduction of a spare pool of memory blocks always ready for writing and to their efficient algorithms of wiping unused cells that worked together with the TRIM command. Most importantly for server applications, you could have all this in a single and rather compact device with very low power consumption.
SSDs soon encountered performance limitations. The conventional SATA interface proved to be too narrow for them, and the number of channels in the multichannel controllers could not be increased infinitely. The solutions were found easily, though. SSDs acquired PCI Express interface which, in its 2.0 version, can transfer up to 0.5 gigabytes per second in both directions along a single lane. The number of memory channels and storage capacity were increased by introducing a RAID controller directly into the SSD design. In other words, such an SSD is actually a composite device in which there are actually multiple SSDs, each with a dedicated multichannel flash controller accessing its own memory chips, hiding behind a RAID controller.
In our previous article we already took a look at one such product and compared it with conventional SSDs. Now we are going to discuss this kind of SSDs in more detail.
OCZ RevoDrive: 50, 80, 120 and 240 GB
The OCZ RevoDrive looks like a RAID controller, especially if you are old enough to remember how RAID controllers had looked before they transformed into today’s highly integrated products with just a couple of electronic chips. It is an expansion card with PCI Express x4 interface that carries two SF1200 flash controllers, a RAID controller Silicon Image SiI3124, a PCI-X à PCI Express converter, and some memory chips. The exact number of the flash memory chips depends on the storage capacity: 32 in the two top-end models, 24 in the 80GB model, and 16 chips in the 50GB RevoDrive. Clearly, the number of memory channels is reduced proportionally in the junior models, which should have an effect on their performance. We will see that in today's tests.
The RevoDrive series does not support TRIM because of the RAID controller but claims to deliver fantastic performance: a read speed up to 540 MBps and a write speed of 400 or 480 MBps (sustained and peek write speed, respectively). Interestingly, you can access the BIOS of the integrated RAID controller and even change its operation mode from RAID0 to RAID1 or JBOD. We did this with one of our samples out of curiosity, effectively cutting the number of memory channels by half. This provides a unique opportunity to see how important the number of memory channels is for an SSD, the total amount of flash memory being the same.