On 5/16/2012 8:24 PM, Oliver Martin wrote: > Hi, > > is there any specific reason why md refuses to create a RAID6 array with > 3 disks? My (probably naive) understanding suggests it should be the > same as a 3-disk RAID1, similar to a 2-disk RAID5. > > The reason I'm asking is that I currently have space on three disks for > a new array, and would like to expand it when I add a fourth. I tried > this scenario with a few loopback devices, but the only way to go from a > 3-disk RAID1 to a 4-disk RAID6 seems to be via an intermediate 3-disk > RAID5, requiring two reshapes. I'd like to avoid one of them, if at all > possible. Make a 3 partition (seems you're using partitions) md RAID 10 (RAID 1E) array with what you have now. When you add a disk down the road, backup your filesystem, create your 4 disk md RAID 6 array, format, restore. Now for my $0.02: You're better served all around with a 4 partition RAID 10 or 1+0, than a 4 partition RAID 6. Random write performance is greatly superior, rebuild time is significantly lower, etc. Sure it would be nice if mdadm could reshape a 3 disk RAID10 into a 4 disk RAID10, but it can't. But since you should be doing a backup/restore anyway, it doesn't matter. -- Stan -- 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
