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Dataram makes SAN flash cache sandwich

Behind the Fibre Channel switches

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Memory supplier Dataram has found another place to stuff flash cache, parking a pile of it between drive arrays and fabric switches in a SAN.

Dataram has been around for ever (well, since 1967) and builds and ships memory products to speed server apps. In 2008 it bought Cenatek, an SSD supplier, and Jason Caulkins, that company's founder, is now Dataram's chief technologist.

In August Dataram said it would deliver "a unique storage product line that will deliver application performance improvements as well as cost savings without the need for an expensive storage upgrade or replacement". XcelaSAN is the product and is a cache, an embedded server with caching software and flash memory, that sits inline between Fibre Channel fabric switches in a storage area network (SAN) and the drive arrays.

What do you get for your money? Dataram is a bit coy. We do know that inside the 2UK enclosure there are eight 4Gbit/s Fibre Channel ports, hot-swappable and redundant power-supplies, 128GB of DRAM, 360GB of flash storage but we don't know if its fast single-level cell or cheaper and less reliable multi-level cell NAND. There are also intelligent caching algorithms.

Caulkins says: "The XcelaSAN is a combination of Dataram memory, flash SSD's and a ton of proprietary software. The software is focused on how to make the XcelaSAN transparent, as well as how to intelligently cache the most frequently used block data."

Dataram says the diskless product uses ChipKill technology which increases DRAM reliability by writing a duplicate copy of data in checksum form to memory. If the primary copy is lost due to a memory failure, single-bit to whole chip, then it's recomputed from the checksum dupe.

Another reliability feature is that in-memory data is written to flash after a power outage, and then the data is automatically restored when power is restored. Management is through a web browser.

We're told that Dataram can upgrade to 8Gbit's Fibre Channel if necessary, and that a 10Gbit/s Ethernet interface or interfaces could be added too.

We could think of this box then as a combined DRAM/NAND cache which avoids the need to have flash drives inside a storage array or a flash accelerator cache in an array's controller. Instead put a SAN array-wide caching or optimising box between the arrays and their Fibre Channel switch.

The box caches the data that is fetched by servers through that switch. If you have more than one switch then you need more than one XcelaSAN box so that you are covered if a switch fails.

Its existence is transparent to accessing servers and backend storage arrays. Both request and receive DTA blocks as before with the XcelaSAN storing previously-requested blocks in its cache and sending them to servers a hundred or more times faster, according to Caulkins, than the back-end arrrays.

He said: "The XcelaSAN is designed to provide an independent, scalable tier zero into existing SAN infrastructure. The key is that we provide all of the benefits of an intelligent tier zero, without changing anything about your existing SAN. In fact, you can re-purpose performance-optimised storage systems to capacity-optimised, and greatly reduce your SAN infrastructure, resulting in lower equipment, maintenance, power, cooling and licensing costs.

"We also scale between vendors, meaning, if you optimize your current SAN, you can change storage vendors completely, and still enjoy the benefit of a solid state optimized system with your new vendor."

The XcelaSAN box is said to do up to 450,000 IOPS and have a 3GB/sec throughput. With this performance Caulkins says that, for a 1TB database application, the product can replace 8 storage arrays containing a total of 200 15K SAS drives, used for speed and not capacity, with a single 25-drive SAS array plus an XcelaSAN sitting in front of it. Dataram says the 200 SAS drives would cost $124,000 to buy and $18,000 to operate over three years.

The XcelaSAN box plus 25 SAS drives would cost $104,000 to buy and $6,000 to run for three years. Customers would save, using this scenario, $20,000 on the purchase costs and $4,000 a year on operating costs.

We've just written about six places in the server-to-external-storage-array stack where you can have flash. Dataram has just rendered that out of date by creating a seventh location.

The XcelaSAN costs $65,000 but is not actually available yet, being still in beta. It will begin US shipping in the fourth quarter and into Europe and Asia in the first half of 2010. ®

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