nixCraft posted a guide about speeding up Linux software raid building and re-syncing

It is no secret that I am a pretty big fan of excellent Linux Software RAID. Creating, assembling and rebuilding small array is fine. But, things started to get nasty when you try to rebuild or resync large size array. You may get frustrated when you see it is going to take 22 hours to rebuild the array. You can always increase RAID resync performance using the following technique.

  HowTo: Speed Up Linux Software Raid Building And Re-syncing