The end of battery-backed cache?

Adaptec has announced the immediate availability of a Series 5Z Unified Serial (SATA/SAS) RAID controller family with "Zero Maintenance Cache Protection." It's designed to replace the current generation of RAID controllers that use lithium ion batteries to protect data in cache memory.

This announcement is significant for two reasons. First, its shows the expanding role of NAND flash. Recently, NAND flash has been associated with the second coming of solid state disk (SSD) in enterprise disk arrays. In spite of reliability issues (the more flash memory is written to, the less reliable it becomes over time), silicon vendors including Samsung and Intel now claim that flash can be made ready for prime-time data center applications by using a "wear leveling" process. Vendors like EMC and Sun have agreed by implementing flash as "Tier 0" storage within their arrays.

Adaptec is now using it for another application: NAND flash coupled with Supercapacitor technology to back up RAID controller cache in case of a system or power failure. RAID controllers use cache memory to accelerate I/O performance between the host system and disk. At any given moment during normal operations, there could be hundreds of write operations waiting to be transferred from cache to disk that. In the case of system or power failure, those could be lost. RAID controller vendors often deal with this exposure by including an optional battery power capability that keeps cache (i.e. RAM) powered-on for between 48 and 72 hours.

Here Adaptec does it a bit differently. It uses 4GB of NAND flash coupled with Supercaps to preserve I/O transactions stuck in cache. In case of a system or power failure, transactions in cache are destaged to NAND, to be "replayed" when normal functioning is returned. In this application, reduced reliability over time is not an issue since the NAND flash would be used in rare occasions, as a failure recovery mechanism.

The elimination of batteries to support cache backup is the second reason this announcement is significant. Engineers who design RAID controllers have kept up with the advance of battery technology over the years. Here, good batteries are those which are small and powerful. Consequently, many have preferred to use the lithium ion variety.

But there is some pain involved. Lithium ion batteries have to be "conditioned" before they are put into productive use. Their operating temperature has to be controlled. They also require periodic testing, as they have a limited life-span. Many fail completely after three years. These shortcomings require that users periodically test the batteries, a process that forces the removal of cache from the I/O path during testing. Finally, vendors who ship products with lithium ion batteries outside the US have to endure a border-crossing "red tape" process which they say can be a royal pain.

With delivery of the 5Z Series, Adaptec has replaced batteries altogether with Supercaps. Like batteries, Supercaps hold a charge over time but work from a different electrical principle. In the 5Z design they are used to power the data de-stage operation from cache to NAND flash storage. Unlike lithium ion batteries, they can be put into productive use immediately, don't need to be checked periodically, and won't get held up when crossing the border.

It's a breakthrough that other vendors of RAID controllers are also working toward. You can expect to see competitors release NAND/Supercap cache backup before the end of the year. For now, Adaptec stands alone. But it is likely that battery-backed cache is about to take a spot in the museum of computing history.

[ahl0003]

Hey John,

While I realize that this wasn't the main thrust of your article, but what we've done at Sun is actually significantly different than using flash as Tier 0. This link may be helpful.

http://blogs.sun.com/ahl/entry/hybrid_storage_pools_in_cacm

Adam Leventhal