Thursday, April 16, 2009

RAID 6 and that (write intent) Bitmap

For the semi-uninitiated, what is the importance of RAID 6 over RAID 5 is the question. According to 3ware (apparently one of the leading hardware vendors for RAID) say that "Double-parity RAID, commonly known as RAID 6, safeguards against data loss by allowing up to two consecutive drive failures". The technology brief continues to say that RAID 5 is vulnerable to two-(2) types of error conditions: all data is lost in the unlikely event of a second drive failure during a rebuild; and latent media errors that are undetected media defects in the same case as a disk failure on rebuild. Essentially the conditions are the same in that it generally means you have multiple failures in your array instead of the single spare disk you have allocated. There are also performance hit differences, but essentially it looks to be that RAID 6 has a 10% write penalty advantage while RAID 5 has a 15%.

Another good resource introduction to the pro's and con's comes from an article in 2007 saying that RAID 6 is here and viable given how cheaper storage is today. I would have to agree given that USB thumbdrives can now go as large as 64-GB which used to be about the size of my laptop hard drive in 2003. With SSD drives now as large as 250-GB its now viable to get instant startup gratification.

So what about Write Intent Bitmaps? A related discussion on QNAP has some pointers to other external information, but its all not definitive.

Some poor chap basically got flamed on the QNAP forum for asking. Is this the current state of the world today where the smart-asses attack the green apples? Would the preventive thing to do such as a QNAP Korea forum help? Or is this just lashing out payback in some way when non-English based support forums are your only hope for this smashingly great device that you bought from overseas and you can't speak a spit of that other language? Hmmm ... I digress.

To me, given the result of turning on the Bitmap option has alot to do with RAID 6. Thats an over-simplification but too much reading and my best guess (and hope!) is that between the 4-TB spare total I'm going to have on the TS-809 would be better justified if it were also used to ensure that parity and the speed of any possible crash rebuild/resync with write intent bitmap turned on. Given that 2TB initial loss to RAID 5, it had BETTER be.

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