Flash and the five-minute rule

Why Now?

Why is this happening now? Dale said you can put flash chips together to get capacity not too expensive compared to disk. Secondly, there is a ton of innovation being poured into MLC NAND, which is cheaper than SLC. Thirdly, array vendors getting much better at tiering and so you can put flash drives as a tier zero in there.

He said, all things considered, "We're at the front end of a big discontinuity," and, getting back to the five-minute rule and comparing DRAM and flash, not DRAM and disk, "You can keep five hours worth of data in flash cache ahead of disk now." When treating flash as backing store for DRAM, the five-minute rule holds at a 6KB page size for DRAM.

The result of this is that you won't buy as much DRAM as you do do now. Also the random working set in DRAM can be reduced from one hour to five minutes.

Enterprise Arrays

Enterprise arrays are affected by this just like host servers. Dale said the application area opportunities include intense random reads, sequential read after random write and low read latency, enabling memory resident apps.

He identified three use cases: flash now showing up as tier zero in arrays, read caching in the controller with the hot working set always there, and having SSD cache in the network. Think Avere.

The flash invasion of arrays means you will buy fewer disks. It's inevitable and it's only the beginning. He thought it will affect vendors more than customers.

Ironically NetApp only has one of these use cases in its product line - caching in the controller with its PAM card - it has yet to introduce tier zero flash into its arrays.

His presentation finished up with a prediction that we will see the introduction of a flash-based Storage Class Memory sitting between DRAM and disk arrays by 2013. He showed a slide predicting that MLC NAND would meet enterprise HDD on cost/GB around 2012-2013. His penultimate slide said:

Over the next five years solid state technologies will have a profound impact on enterprise storage ... The architectural balance of memory, cache and persistent storage will change. Today's solid state implementations in enterprise storage demonstrate these changes. It's only the beginning.

There is a storage discontinuity between DRAM and hard drives and flash is going to fill it - that is now beginning to look like a certainty.

Bootnote

David Dale's presentation is on an SNIA website for the London event. If you are in the SNIA you can request the location and password by emailing Paul dot Trowbridge at Evito dot com. If you are not in the SNIA then we guess you'd better join and then send the email to Paul. ®