On Tuesday January 24, lmb@xxxxxxx wrote: > On 2006-01-24T11:40:47, NeilBrown <neilb@xxxxxxx> wrote: > > > I am expecting that I will ultimately support online conversion of > > raid5 to raid6 with only one extra device. This process is not > > (efficiently) checkpointable and so will be at-your-risk. > > So the best way to go about that, if one wants to keep that option open > w/o that risk, would be to not create a raid5 in the first place, but a > raid6 with one disk missing? > > Maybe even have mdadm default to that - as long as just one parity disk > is missing, no slowdown should happen, right? Not exactly.... raid6 has rotating parity drives, for both P and Q (the two different 'parity' blocks). With one missing device, some Ps, some Qs, and some data would be missing, and you would definitely get a slowdown trying to generate some of it. We could define a raid6 layout that didn't rotate Q. Then you would be able to do what you suggest. However it would then be no different from creating a normal raid5 and supporting online conversion from raid5 to raid6-with-non-rotating-Q. This conversion doesn't need an reshaping pass, just a recovery of the now-missing device. raid6-with-non-rotating-Q would have similar issues to raid4 - one drive becomes a hot-spot for writes. I don't know how much of an issue this really is though. NeilBrown - 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
