On the ESS the write cache is not optional.  You must use it.  That is where
all the write performance comes from.  It eliminates the RAID-5 Write
penalty and basically changes the writes to RAID-3, no reads before write,
when writing sequentially.

Not sure what you mean by 2 write cycles.  RAID-1 has two write cycles.
RAID-5 has one write cycle to all the drives in the stripe set unless all
the parity is on a single drive, but no one does it that way anymore (EMC
kind of does with RAID-S).  The issue with RAID-5 is the read before write
so that you can recalculate the parity.  On sequential write of full
internal 32K blocks (6 or 7 depending on the disk group) you do not need to
read back because you are going to write the whole stripe.  So, the ESS
calculates the parity in the SSA adapters and writes it to the disks as the
IO occurs.

The rest of the stuff in here is really good stuff.  Though backupsets are
extremely hard to manage for customers with only 3494/3590 libaries because
the client cannot do the restore.  The reality is they run so fast anyway
many of the issues that backupsets eliminate do not necessarily apply if you
use collocation and parallel restores of them.