For the most part you lose nearly 1/3rd of total space for RAID5 and due to higher levels of compression with RAID 6, it's slightly more due to the fact that the Parity structure is doubled. Raid 5 requires a minimum of 3 drives and Raid 6 requires 4, the compression algorithm's get better with the more drives you add, especially with RAID 6, but RAID 6 will always use more space for Parity than RAID 5, but less than RAID 10, so it's a good balance for those who want to have 4 drives or more with better protection than RAID 5 without losing quite as much space.