In order to get the best performance from your RAID storage, you need to align everything (the filesystem with your RAID setup)!
1 - chunk size of raid array - how you choose this value is not under this post's scope, let's consider the default for my raid controller - 256k
2 - filesystem block size (ext4) is set to default - 4k
3 - filesystem stride = array chunk size / filesystem block size: 256k / 4k = 64k
4 - filesystem stripe-width = filesystem stride * number of data disks in array (for a raid5 with 4 disks, this will be 3): 64k * 3 = 192k
So this will translate into:
$ mkfs.ext4 -b 4096 -E stride=64 -E stripe-width=64 /dev/sdb