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IBM multi-petabyte cloud defies XIV storage

How does it scale that much?

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Comment With its Smart Business Storage Cloud, IBM says we have its GPFS and XIV being used to build a system capable of multiple petabytes of capacity and supporting billions of files with high-performance computing-like I/O performance. But XIV has a maximum usable capacity of 79TB.

We can envisage an IBM BladeCenter server set running the GPFS (General Parallel File System) connected to an XIV rack, but this seems utterly unbalanced. There is a BladeCenter/GPFS setup capable of handling multiple petabytes and billions of files on the one hand, hooked up to an XIV array that can only hold 79TB and hundreds of thousands or single digit millions of files of any size on the other. What is going on? How will this work?

To get XIV storage up to the multi-PB level IBM has either to scale up XIV, scale out XIV, or do it indirectly and scale out server nodes with attached XIV arrays. Which is it going to use?

Scaling up XIV

XIV arrays use 1TB drives. Let's bring in 2TB drives and make the capacity 158TB. It's still nowhere near enough. If we assume a multi-petabyte capability means 10PB then we would need 64 times as many drives as it could use today. That means a 120X increase in capacity as we continue scaling towards 20PB.

It seems extremely unlikely that IBM is going to announce a souped-up XIV product with a 64X or 120X increase in capacity. The backplane and controller-drive enclosure interconnect technology would be awesomely difficult to design, engineer and develop.

Scaling out XIV

How about clustering XIV arrays? Can we do that? The XIV, with its 21 nodes, is already internally clustered, IBM describing it thus: "The XIV system is based on a grid of standard, off-the-shelf hardware components connected in any-to-any topology by means of massively paralleled, non-blocking Gigabit Ethernet." Devising a cluster interconnect and node software to keep things on track for an already clustered product is like engineering a cluster of clusters.

To scale to, say, the 10PB level, an XIV array using 2TB drives and maxing out at 158TB would need 64 nodes. A 64-node interconnect capable of scaling to higher node levels is perfectly feasible.

Isilon and Exanet make NAS clusters that go past the 1PB capacity level. Isilon can have up to 96 nodes in a scale-out design using 20Gbit/s InfiniBand. So we could envisage some way of scaling out XIV capacity by linking XIV nodes together with InfiniBand and adding functionality to XIV software to enable the nodes to co-operate.

However, scaling up XIV in this way would need a lot of XIV development work. Suppose we take a different tack.

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