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 know a 1+2 disk raid6 sounds stupid but it has an use case. Let me > demonstrate this with raid5: > > Lets say you started with a raid1 when you bought your computer: > mdadm --create -l 1 -n 2 /dev/md9 /dev/sda1 /dev/sdb1 > a fewyears of use on /dev/md9 > > Now you run out of space and went and bought a 3rd harddisk. What to > do now? > mdadm --stop /dev/md9 > mdadm --create -l 5 -n 2 --assume-clean /dev/sda1 /dev/sdb1 > mdadm --add /dev/md9 /dev/sdc1 > mdadm --grow -n 3 /dev/md9 > > Voila. A nice raid5 with twice the size of the raid1. And no single > point of failure like degrading the raid1 and copying the data to a > degraded raid5 would have. And the downtime is just a minute to create > the new superblock. In 2.6.30 and later you wont even need the downtime. Just echo raid5 > /sys/block/md9/md/level in place of the "stop" and "create" steps. > > > > Now for the raid6 case. With only 1 data disk and 2 parity disks all 3 > disks should end up with identical data on them. In effect this should > be a 3 disk raid1, a cpu intensive one. Take an existing raid1 with 2 > or 3 disks, stop the raid, create a new raid6 ovver it with > --assume-clean, start the raid. After that one can add more disks and > --grow -n 4/5/6/.. the raid6 to a sensible size. Again without going > into degraded mode. > > > So back to my original question: Why does the kernel require 4 disks > for a raid6 instead of allowing 3? I have occasionally wondered that. But I didn't write that code and never saw a need to change it. As Andre says, it is very likely that just relaxing the restriction will allow it to "just work". Maybe that will happen in 2.6.31... 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
