Q:

mdadm RAID Fast Setup with Empty Drives?

I am recreating some RAID5 disks as RAID6 with mdadm.  There is no data on the drives that I care about.
Setting up RAID takes a while to setup the shape - I accept that when there there is data that needs to be striped and parity calculated, however with these drives, they are empty - or at least I want them to be considered empty.
So is there a way to skip the parity calculation and tell mdadm to just set up the superblocks and be done, or otherwise, what exactly is it spending all this time on when there is no data to move around?
md3 : active raid6 sdf3[5] sde3[4] sdd3[3] sdc7[2] sdb3[1] sda3[0]
      1953114112 blocks super 1.2 level 6, 512k chunk, algorithm 2 [6/6] [UUUUUU]
      [>....................]  resync =  1.3% (6790144/488278528) finish=409.3min speed=19604K/sec

Note that I am not talking about --assume-clean where you are rebuilding an array from a pre-existing set of disks that contain a RAID array that you know is correct.  I am talking about an array that should be considered empty, not considered correctly striped.
So lets say for the sake of this question that the devices have been pre-populated with zeros.

A:

You can use --assume-clean but unless you are using raid5 ( not raid6 ) and the disks actually are full of zeros, the first time it runs a parity check, it will come up with errors that will need corrected, so you should not do this.  You don't need to wait for the resync to finish before you can start using the array; it will chug along in the background until it is done.

