Thursday, January 14, 2016

Developing Memory Trends

There are a few new types of memory breaking onto the scene that are reportedly going to change the world.  The trick to bringing a memory technology to the commercial market today is nailing all 4 key properties: fast, dense, non-volatile, and inexpensive.  
  1. Normal DDR3 DRAM is fast (6-20 nanoseconds) and dense, but volatile and expensive at $50/GB for enterprise server DRAM.
  2. NAND flash is dense, non-volatile, and inexpensive (~$5/GB for enterprise SSD), but it's nowhere near the speed of DRAM at >50,000 nanoseconds
  3. Memristors sound amazing, but they don't exist yet and likely won't in the next 5 years.  
  4. 3D Xpoint appears to be the the only viable option right now.  Intel and Micron report it is:
    1. Fast: 1,000x faster than Flash would mean 50 nanosecond range.
    2. Non-volatile
    3. Up to 50% less expensive than DRAM
    4. 10x denser than DRAM
Importantly, 3Dxpoint is reported to be durable.  One of the drawbacks of Flash is writes cause damage to it over time, so if you start with a 1TB disk after 4 years you may have a 200GB disk.  To solve this, manufacturers pack up to 6x the needed amount of Flash into an SSD so when an area goes bad, the SSD just allows you to write to a brand new area of the disk.  This does two bad things: drives up cost and drives down performance. 
"We show that future gains in density will come at significant drops in performance and reliability. As a result, SSD manufacturers and users will face a tough choice in trading off between cost, performance, capacity and reliability."  Source
If 3dxpoint delivers on its promises, there will be several huge impacts.
  • All-3dxpoint arrays.  Today's storage operating systems will need to be completely re-written as they are simply not capable of going from 50,000ns disk latencies to 50ns.  
  • All systems will have more memory.  At 50% the cost of DRAM, rather than spend less money on memory we'll probably just spend the same money on 2x the amount of memory.
  • Because you'll have 2x the memory, operating systems will need to be re-written.  Our current OS's are designed around the cost constraints of memory gradually declining according to Moore's law.  3dxpoint would thrust us forward along that line and require serious software engineering to take advantage of it.
  • Since it's non-volatile, operating systems will need to be re-written.  Lose power?  Start right back where you left off.  This means you wouldn't be able to resolve an application/OS freeze-up with a hard reboot as well.
  • FaME (Flash as Memory Extension) will give way to 3dxpoint as Memory Extension and accelerate the trend rapidly.  The cost of 3dxpoint ($25/GB?) make a strong case for the shared-resource model of today's data storage industry, while the performance and non-volatility feature would be the succession of SAP HANA's "database in memory" architecture (and IBM's Spark, too).  
DRAM's performance advantage over 3dxpoint probably means DRAM won't go away in the enterprise.  Rather, 3dxpoint would be another tier of memory, below DRAM and above disk, with DRAM mirrored to the 3dxpoint for its non-volatility.  

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