Category Archives: Data Corruption

Apple chomps Anobit

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A few days ago, Apple paid USD$500 million to buy an Israeli startup, Anobit, a maker of flash storage technology.

Obviously, one of the reasons Apple did so is to move up a notch to differentiate itself from the competition and positions itself as a premier technology innovator. It has won the MP3 war with its iPod, but in the smartphones, tablets and notebooks space, Apple is being challenged strongly.

Today, flash storage technology is prevalent, and the demand to pack more capacity into a small real-estate of flash will eventually lead to reliability issues. The most common type of NAND flash storage is the MLC (multi-level cells) versus the more expensive type called SLC (single level cells).

But physically and the internal-build of MLC and SLC are the exactly the same, except that in SLC, one cell contains 1 bit of data. Obviously this means that 2 or more bits occupy one cell in MLC. That’s the only difference from a physical structure of NAND flash. However, if you can see from the diagram below, SLCs has advantages over MLCs.

 

NAND Flash uses electrical voltage to program a cell and it is always a challenge to store bits of data in a very, very small cell. If you apply too little voltage, the bit in the cell does not register and will result in something unreadable or an error. If you apply too much voltage, the adjacent cells are disturbed and resulting in errors in the flash. Voltage leak is not uncommon.

The demands of packing more and more data (i.e. more bits) into one cell geometry results in greater unreliability. Though the reliability of  the NAND Flash storage is predictable, i.e. we would roughly know when it will fail, we will eventually reach a point where the reliability of MLCs will no longer be desirable if we continue the trend of packing more and more capacity.

That’s when Anobit comes in. Anobit has designed and implemented architectural changes of the way NAND Flash storage is used. The technology in laymen terms comes in 2 stages.

  1. Error reduction – by understanding what causes flash impairment. This could be cross-coupling, read disturbs, data retention impairments, program disturbs, endurance impairments
  2. Error Correction and Signal Processing – Advanced ECC (error-correcting code), and introducing the patented (and other patents pending) Memory Signal Processing (TM) to improve the reliability and performance of the NAND Flash storage as show in the diagram below:

In a nutshell, Anobit’s new and innovative approach will result in

  • More reliable MLCs
  • Better performing MLCs
  • Cheaper NAND Flash technology

This will indeed extend the NAND Flash technology into greater innovation of flash storage technology in the near future. Whatever Apple will do with Anobit’s technology is anybody’s guess but one thing is certain. It’s going to propel Apple into newer heights.

Silent Data Corruption (SDC) …it’s more prevalent that you think

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Have you heard about Silent Data Corruption (SDC)? It’s everywhere and yet in the storage networking world, you can hardly find a storage vendor talking about it.

I did a paper for MNCC (Malaysian National Computer Confederation) a few years ago and one of the examples I used was what they found at CERN. CERN, the European Center for Nuclear Research published a paper in 2007 describing the issue of SDC. Later in 2008, they found approximately 38,000 files were corrupted in the 15,000TB of data they generated. Therefore SDC is very real and yet to the people in the storage networking industry, where data matters the most, it is one of the issues that is the least talked about.

What is Silent Data Corruption? Every computer component that we use is NOT perfect. It could be the memory; it could be the network interface cards (NICs); it could be the hard disk; it could also be the bus, the file system, the data block structure. Any computer component, whether it is hardware or software, which deals with the bits of data is subjected to the concern of SDC.

Data corruption happens all the time. It is when a bit or a set of bits is changed unintentionally due to various reasons. Some of the reasons are listed below:

  • Hardware errors
  • Data transfer noise
  • Electromagnetic Interference (EMI)
  • Firmware bugs
  • Software bugs
  • Poor electrical current distribution
  • Many more …

And that is why there are published statistics for some hardware components such as memory, NICs, hard disks, and even protocols such as Fibre Channel. These published statistics talk about BER or bit-error-rate, which is the occurrence of an erroneous bit in every billion or trillion of bits transferred or processed.

And it is also why there are inherent mechanisms within these channels to detect data corruption. We see them all the time in things such as checksums (CRC32, SHA1, MD5 …), parity and ECC (error correction code). Because we can detect them, we see errors and warnings about their existence.

However, SILENT data corruption does not appear as errors and warnings, and they do OCCUR! And this problem is getting more and more prevalent in modern day disk drives, especially solid state drives (SSDs). As the disk manufacturers are coming out with more compact, higher capacity and performance drives, the cell geometry of SSDs are becoming smaller and smaller. This means each cell will have a smaller area to contain the electrical charge and maintain the bit-value, either a -0 or -1. At the same time, the smaller cell is more sensitive and susceptible to noise, electrical charge leakage and interference of nearby cells as some SSDs has different power modes to address green requirements.

When such things happen, a 0 can look like a 1 or vice versa and if the error is undetected, this becomes silent data corruption.

Most common storage networking technology such as RAID or file systems were introduced during the 80’s or 90’s when disks were 9GB, 18GB and so on, and FastEthernet was the standard for networking. Things have changed at a very fast pace, and data growth has been phenomenal. We need to look at storage vendors’ technology more objectively now and get more in-depth about issues such as SDC.

SDC is very real but until and unless we learn and equip ourselves with the knowledge, just don’t take things from vendors verbatim. Find out … and be in control of what you are putting into your IT environment.