I actually calculate it as 1.5MB (1536K).

To be clear let me use the SNIA definition of “stripe size” as a baseline –

“A parity RAID array's stripe size is the stripe depth multiplied by the number of member extents less the number of parity extents”

So lets break this down…..

Stripe depth on the USP and USP V for OPEN-V volumes is 512K.  

Member extent is a fancy way of saying member disks. In your example there are 4 member disks (3+1=4). So member extents = 4.  

Parity extent is a fancy way of saying parity disks*, which in your 3D+1P is 1. So parity extents = 1

So the calculation is -

512 x (4-1) = 1536

It is true that there are 4 disks in your RAID5 (3+1) configuration and 4 x 512 = 2MB. However, you need to minus the parity to get the true stripe size

Below is a list of all stripe sizes for the USP and USP V

RAID1 (2+2) = 1024K

RAID1 (4+4) = 2048K

RAID5 (3+1) = 1536K

RAID5 (7+1) = 3584K

RAID6 (6+2) = 3072K

RAID5 (14+2) = 7168K

RAID5 (28+4) = 14336K

*Of course there is no dedicated parity disk as my explanation above might imply, the parity extent rotates across all disks.

Hope this helps and feel free to ask any more questions.

Ive put a couple up, but its nearly mifnight here in England so I promise to put the rest up over the weekend.

Mind you Im a storage guy not an artist so they wont be no Michaelangelos'