On Fri, Jul 5, 2013 at 11:20 AM, Ben Scott <[email protected]> wrote: > On Fri, Jul 5, 2013 at 1:37 PM, David Lum <[email protected]> wrote: >>> Be careful of a SATA Raid array. I've got a couple of these and in an >>> effort to save money and I'm not doing it again. The issue comes down to >>> something I learned about the hard way. "Array Puncture"... >> >> I have never heard this term before today, looking it up now. > > Me neither. This explains it: > > http://www.dell.com/support/troubleshooting/us/en/04/KCS/KcsArticles/ArticleView?c=us&l=en&s=bsd&docid=438291 > > Nothing to do with SATA, it appears. > > It's just a fallback method for a double fault. A double fault is > when two blocks (stripes) are both found to be bad. This is more than > a RAID 5 can tolerate. A "puncture" appears to just be an option to > keep an array operating despite a double fault. You've got two > choices when you encounter a double fault, fail the entire array > offline, or "puncture" it, rebuilding what you can, and returning > error on those particular blocks. > > The double fault problem has been known for decades, and is why > better RAID implementations do regular consistency checks across all > disk members. > > -- Ben
Two things to add to that: o- RAID 6 is more resilient, but of course the controllers cost more, and there is a performance penalty vs. other RAID categories, which better ($) controllers can mitigate. o- Larger arrays (whether made up of larger disks or larger numbers of disks, or both) will have more errors, and therefore a higher probability of simultaneous errors, than smaller arrays. They are also slower to perform consistency scans against. So, having said that, I'm of the opinion that a RAID 5 array of 16 x 300gb drives is probably OK for RAID 5, especially if one of them is a dedicated hot spare. I haven't done the math to back that up, but in the world of 3.5" drives it's been pretty reasonable. Don't know about OPs situation, as he's considering 2.5" drives, though a quick search of CDW reveals that 300gb, 2.5", 10k rpm SATA drives don't exist - although they have them in SAS and SAS-2, in both 10k and 15k speeds. Kurt
