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.

Solution 1:

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.

Solution 2:

You can't do this with a software or hardware RAID. All checksums need to be written on the disks, which takes time. You can do it later, but then the parts of the disk that isn't written to, will have to do that before you can use them.

This is basicly because the RAID system and file systems doesn't know a thing about each other. ZFS has a sollution for this, but there the RAID parts are deep integrated with the file system. So the RAID subsystem actually knows what parts of the disks are used to store data on and which can be used later and then write the check sums to them.

You can add throughput speed to the software RAID or you start using the RAID before all checksums are written, and let the software RAID handle this for you later. Witch is what @psusi wrote.