Nifty Fedora Mitch <niftyfedora@xxxxxxxxxxxx> writes: >> On Tuesday March 17, goswin-v-b@xxxxxx wrote: >> > Hi, >> > >> > I'm wondering why the kernel requires a raid6 to have at least 4 >> > disks (of which at most 2 can be missing). Why not 3 disks? > > I should think that this is the 'defination' of a raid6. > If you build a raid6 resource it should be raid6 and > as such be able to tolerate the loss of two disks > and all the other raid6 properties. Actually the definition I read was that raid6 is like raid5 but allowing for 2 or more disks to fail without loss. So a 15 disks raid with 10 data blocks and 5 parity blocks per stripe would also be raid6. > If you build a raidN with three disks that can tolerate the > loss of one disk it is not raid6 but raid5 and should be called > by the correct descriptive name. I was talking about a raid6 with three disks that can tolerate the loss of *two* disks. I know that effectively that would be just like a raid1 with 3 disks but that was exactly the point. To change a 3 disk raid1 into a 3 disk raid6 and then grow it. > The importance of this strictness surfaces in documentation. > Both to the system admin working on the system and also > documentation from the system admin to his management that > is comparing prices and matching features. i.e. "Bob's raid6 > is 25% less costly than Fred's". Truth in advertising comes > to play. > > Hidden in this thread is an interesting notion of migration from a > "lesser" raid to a more durable raid over time. It might make sense to > facilitate limited tools to this end. Even for those sites that wish to stage > the construction of a large raid install or to stagger the poweron hours > and date codes of drives on the common notion that batches of drives > fail together. Or perhaps a "raid6 ready" raid5 that gets populated with > the absent drive on the first statistical sniff of an error outside of > the norm. Or the arrival of warrenty expiration.... That would be a raid6 with 4 disks of which one is missing. You can already do that. > Such a "raidN ready" setup does confuse the notion of operational health > for any monitoring tool and is likely a very bad idea for that reason alone. > > This 2001 URL is interesting in the comments about how unfortunate the > choice of the world "level" was. > http://www.pcguide.com/ref/hdd/perf/raid/levels/index.htm > also interesting is the coment about a vendor being sloppy > with the technical language. Verry badly advertising polluted url and confusing imho. http://en.wikipedia.org/wiki/RAID MfG Goswin -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
