SSD Reliability

• storage reliability - includes articles about reliability for all media (not just SSDs)

• Can you trust your flash SSD specs? - the product which you carefully qualified may not be identical to the one that's going into your production line, because the SSD oem has "improved" it. But the improvement makes another operating parameter - which you deeply care about - unacceptably worse.

• Flash SSD Reliability (pdf) - explains how Texas Memory Systems have engineered reliability into their multi-terabyte flash SSDs using over-provisioning, RAID, wear-leveling etc.

• SSD Myths and Legends - "write endurance" - helped the market re-evaluate one aspect of flash SSD reliability

• sweetening for the enterprise - discusses the different flavors of SLC, eMLC and MLC and the competing management schemes which transform unreliable flash memory chips into reliable enterprise storage.

• Data Integrity Challenges in flash SSD Design - article by SandForce explains how enterprise SSD designers can reduce the risk of "silent errors."

• what happens in SSDs when power goes down? - surveys different designs of power management architectures and discusses their consequences.

• Flash SSD Data Reliability and Lifetime (pdf) - written by Imation - starting from a description of floating gates and going all the way up to the architecture of a flash SSD this paper includes good descriptions of data failure modes, including:- erase failure, (erase) stress induced long term leakage, disturb faults, and the potential for inadequate error correction code coverage in MLC.

• Noise Damping Techniques for PATA SSDs in Military Systems (pdf) - written by SiliconSystems - useful reminders for PCB designers about good design techniques for electromagnetic compatibility to reduce spurious data corruption in PATA SSDs.

• "NAND Flash Solid State Storage for the Enterprise - an in-depth Look at Reliability." (pdf) - published as part of SNIA's SSD initiative - this article is co-authored by:- Jonathan Thatcher Fusion-io, Tom Coughlin Coughlin Associates, Jim Handy Objective Analysis and Neal Ekker Texas Memory Systems.
  
  The article contains the best integrated explanation I've seen of the design trade-offs for error correction schemes and how they affect bit error rates compared to the raw uncorrected results. It goes on to explain the importance of the SSD controller and memory architecture (dispersing data among many chips) and how these can improve data integrity by managing read disturb errors. It also discusses wear-leveling and write amplification which have been well covered elsewhere.

• Why Raw NAND Flash with Hardware-based ECC is the Way to Go - extract - "Error rates are increasing substantially as flash manufacturers push the limits of physics. Errors can be introduced externally by heat or other radiation, during writes or reads of data, and even to data that was successfully written at a different time."

• Why Consumers Can Expect More Flaky Flash SSDs! - Why is it already so bad? Why will it get even worse?

• Real World Reliability in High Performance Storage (pdf) - by Density Dynamics - compares real world failure rates for HDDs and flash SSDs with predicted MTBF and endurance data and suggests that the big discrepancies reported by users are due to the nature of their workloads. In this respect it suggests RAM SSDs are better in heavy IOPS apps - even taking into account the MTBFs of batteries and UPS like components. It also cites my own article RAM Cache Ratios in flash SSDs.

• Increasing Flash SSD Reliability - this classic article by (published in 2005) remains a good read today. Here's the original editor's intro:-
  
  SSDs, based on flash technology, have greatly improved in performance in recent years and now compete head to head with RAM based accelerator systems. Flash also has significant advatanges in servers compared to RAM SSDs due to low power consumption. But if you think that all solid state disks which use flash are equally reliable and enduring then think again. That's a bit like saying that a Mercedes 300SL sports coupe is as tough as a Tiger tank because both were made in Germany and both are built out of metal. But as Oddball (Donald Sutherland) says in the movie Kelly's Heroes "I ain't messing with no Tigers." This article by SiliconSystems, shows how their patented architecture cleverly manages the wear out mechanisms inherent in all flash media to deliver a disk lifetime that is about 4 times greater than of other enterprise flash products and upto 100 times greater than intrinsic flash memory.

• Consistency Groups: The Trouble with Stand-alone SSDs - by Woody Hutsell (published March 2011) discusses different approaches to maintaining data in SSDs in the event of an SSD failure. Some approaches - while simple to implement - can have a large negative impact on performance.

• Flash Solid State Disks - Inferior Technology or Closet Superstar? - this is another classic article by BiTMICRO (published February 2004).
  
  This article was one of many pioneering communications from the flash SSD market to get users to think about flash in a different way. Its main message was - "A general perception in the computing industry is that only DRAM is robust enough for enterprise use. That sentiment doesn't give enough credit to flash memory."

• NAND Flash Memories for Spacecraft (doc) - written by Phil White, President of ECC Technologies is based on a document originally written for NASA and JPL - who have employed these techniques in 2 missions which are already operating in space.

• Signal Processing and the evolution of NAND flash memory (pdf) - written by Anobit's chief scientist Naftali Sommer (published December 2010) describes the role of DSP technology in improving the integrity of logic states read from flash cells.

• Flash Memory Failure Analysis (ppt) - (published November 2007) by Purnima Vuggina Intel - outlines common physical causes of failure in flash memory. It also describes the problems of failure analysis, and future challenges.