Is flash a cache or pretend disk drive?

El Reg: How should HDD-based storage arrays be flash-enabled, either by adding tier zero flash like Symmetrix or by adding a separate SSD enclosure/array like IBM's Quicksilver?

Rick White: The beauty of the IBM approach is that there were no additional enclosures. The SSDs went directly inside the storage array appliances (which were just X-series servers). The PCIe-attached ioDrives could just as easily be placed inside of the existing EMC array appliances, as they too have PCI.

I'd say directly attaching NAND on the PCIe bus can easily be considered a superior approach. The strategy requires no additional enclosure and offers higher performance at a significantly lower cost. This is especially true when considered in relation to the fact that it's solid state and doesn't need access…

El Reg: What do you think of the issue that server operating systems' disk I/O pattern is ill-suited to SSD use? Can any disadvantages be worked around and how?

Rick White: Yes, while most SSDs have a difficult time with random writes (or even sequential writes for that matter), the ioDrive excels in this regard. Again, by not trying to force NAND flash to look like a disk where it doesn't matter (to the sheet metal, SCSI / SATA bus & controller), but only where it does matter (to the OS and software), the ioDrive provides greater degrees of freedom in solving this problem.

To put it more simply, by allowing NAND to have a HW interface that accentuates its strengths, one can better cover the weaknesses of the medium and have it simultaneously appear as traditional block storage to the OS. To back this up, I think a single ioDrive that can do over 500 MB/s write speeds on Microsoft's Vista operating system should answer the question on whether an advanced controller design can work around existing disadvantages or bottlenecks in an operating system.

El Reg: What is Fusion-io's take on MLC flash and also on write endurance cycle lengthening?

Rick White: MLC will play an important role in enterprise SSDs. Its endurance can be extended through various means, but for many vendors it will still be somewhat limited to non-write-intensive applications. Even so, the cost of MLC is just too compelling to not use it, especially by those who can bound their write-load.

El Reg: What is Fusion-io's view on HDD storage array I/O internal and external I/O structures and how it can cope with SSD I/O patterns? What does this mean for storage array vendors?

Rick White: Object-based storage, which allows for content-aware storage media handling, will greatly enhance SSD performance. This ultimately means even tighter software/hardware integration, which is favoured by directly PCIe attached HW; that includes software drivers that can convey this information.

Fusion's ioDrive is actually an Object Storage device that can run in a simplified mode to support traditional dumb block storage. Enhancements to operating systems and file systems to further exploit its object storage capabilities are underway.

Comments

Fusion says to use flash as a large cache directly attached to a storage controlling x86 server's main bus. Put this in a SAN so that it becomes just another piece of shared block-level storage. This PCIe-connect idea certainly seems viable, with IBM’s Quicksilver demonstrating just that. It does mean though that vendors can’t use an approach of dropping 3.5-inch form factor SSDs and controllers into, say, Symmetrix and Clariion hard disk drive enclosures, as EMC has done. White doesn't see this as an issue.

It will be very interesting to see if any storage array vendor picks up this idea and brings out an array with disk drives connected by a Fibre Channel Arbitrated Loop to a main x86 server/controller and a separate stash of Fusion-io SSDs connected to that controller’s PCIe bus. Balancing I/O inside the overall array and providing a fast enough I/O pipe to it might be interesting challenges. ®