Resume mdadm array build after power outage?

Question

I was building a 2tb x 4 disks using mdadm. Originally mdstat said it was going to finish in 48 hours but unfortunately at 47 hours a power outage came into play :-)

So I decided to zero out the superblocks using mdadm and recreate the linux raid partitions to start clean from the beginning. To my surprise this time mdstat said it was only going to last 5 hours. I stopped the build a repeated the zerosuperblock/repartition process thinking I did something wrong but mdstat keeps saying it will only last 5 hours.

I don't understand what Is really happening, is mdadm resuming the Raid build process? Or somehow is re-using so something from he first build and that's why it is way faster? Is My Raid going to be fine?

I have been looking for an answer over the internet with no luck, any help/clarification will be appreciated Thanks!

Answer 1

Well, the power outage should not have ruined things. I would have expected the reconstruction to pick up where it left off as soon as you did the --assemble, and do the remaining 1 hour of work.

I think you wasted 47 hours of work by zeroing things out.

As for the 48 -vs- 5 hours, perhaps before the reboot you had processes contending for IO bandwidth and it was slowing down the synchronization. Now that you have rebooted, those process are not (yet) running, so the reconstruction is running faster.