With RAID1, there is always a second write for mirroring performed at the disk level.  With RAID5 or RAID6, some of the disk overhead with sequential writes can be optimized in cache, if the entire raid stripe is "dirty" in cache.  So for example, if a host writes a full 7+1 RAID5 stripe in cache, and it hasn't been destaged, we can do the XOR calculations in cache, then write 7 sets of data stripes and only have the overhead on disk of writing one parity stripe.  So one extra write for 7 data writes, where RAID1 would have 7 extra writes for the 7 data writes.
In the VMAX solution Design study guide , in the " configuring VMAX for Performance section" it says " Sequential writes on a RAID 5 and RAID 6  can be more efficient than RAID 1",I was a bit surprised reading this, can anyone explain how is this possible ?
Sequential writes on RAID 5 more efficient that writes on RAID 1