Peter L. Ashford wrote: > >>I'm playing around with RAID-6 algorithms lately. With RAID-6 I mean >>a setup which needs N+2 disks for N disks worth of storage and can >>handle any two disks failing -- this seems to be the contemporary >>definition of RAID-6 (the originally proposed "two-dimensional parity" >>which required N+2*sqrt(N) drives never took off for obvious reasons.) > > This appears to be the same as RAID-2. Is there a web page that gives a > more complete description? > http://www.acnc.com/04_01_06.html is a pretty good high-level description, although it incorrectly states this is two-dimensional parity, which it is *NOT* -- it's a Reed-Solomon syndrome. The distinction is critical in keeping the overhead down to 2 disks instead of 2*sqrt(N) disk. RAID-2 uses Hamming code, according to the same web page, which has the property that it will correct the data *even if you can't tell which disks have failed*, whereas RAID-3 and higher all rely on "erasure information", i.e. independent means to know which disks have failed. In practice this information is furnished by some kind of CRC or other integrity check provided by the disk controller, or by the disappearance of said controller. -hpa - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
