Analysis

posted on 19 August 2008 18:39

HP has persistently refused to identify the switching fabric used inside its Extreme Data Storage ExDS9100 product. But an HP document gives the game away; it's an end-to-end SAS switching fabric.

The ExDS9100 is an extremely scalable file storage system based on storage blocks with 82 SAS drives, and performance blocks, with up to 16 HP blade servers, running Linux and HP PolyServe software in a system designed for extreme scalability and the storing of massive amounts of data from Web 2.0-type busineses and the like.

At product announcement time, Michael Callahan, HP's chief technologist for the StorageWorks NAS division, said: "All blades have simultaneous access to all of the storage in the device." But he wouldn't reveal how this was done except to agree that a point-to-point scheme was impractical.

On being asked about this secret switching fabric, HP's StorageWorks VP for EMEA, Neal Clapper, replied: "... yes, we need something to connect the servers and the disks. However, we are not able to divulge how we are doing this since we are too far from product release to give these types of details."

The product is due to be launched before the end of the year, which makes Clapper's assertion a bit of a nonsense.

A close reading of the HP StorageWorks 9100 Extreme Data Storage System fact sheet reveals: "Storage blocks, including 82 serial-attached SCSI (SAS) drives in seven-rack unit space and RAID 6 protected with dual SAS controllers. The SAS switch delivers an always-on infrastructure that requires no additional external switches."

This SAS switch mention isn't repeated in any other HP ExDS9100 literature.

What is a SAS switch? External to what? It must mean external to the storage block. So each storage block comes with a SAS switch. Why? Logically so it can connect to and participate in a SAS fabric.

Serial-attached SCSI is, like Fibre Channel, is based on the SCSI protocol. A SAS switch or expander allows a server (host and SAS initiator) to connect to multiple SAS targets (hard drives) and is the means whereby servers can have an end-to-end connection to SAS storage devices. LSI Logic introduced its Lynx3090 SAS switch in March 2006 with 9 x 4-lane SAS ports. Its copper cable connects could be 8m long and so it was intended for an in-rack fabric - LSI used the fabric term. It supported zoning and you could connect multiple switches to increase the size of the fabric.

The gleam in people's eye then was that a SAS fabric could replace a much more expensive Fibre Channel fabric. Alas, it didn't come to pass.

LSI followed the Lynx3090 with its first generation integrated circuit (IC) SAS expanders, e.g. here. It now has a second generation 6Gbit/sec SAS Expander IC product. Other SAS switch suppliers include PMS-Sierra and Rancho Sys Tech, an HP development partner, with a 36-port SAS switch. It is described thus: "This expander provides large storage environments the ability to connect multiple targets and initiators through a switched device for scalability and fault tolerant path redundancy to improve reliability."

What this has brought us to is this SAS fabric scheme: it will have the Blade servers in the ExDS9100 talk via SAS HBAs (host bus adapters) to, most probably, a base SAS switch which is hooked up to each storage block's SAS switch and through it to two SAS controllers in the block and, through them, to the 82 drives inside. The switch fabric itself is managed via an out-of-band Ethernet link.

In effect the ExDS9100 implements a SAS SAN (storage area network) which the PolyServe software uses for file storage. This is the only end-to-end SAS fabric application we are aware of in a combined server-storage system.

^{[Chris Mellor.]}