The Zero Page Reclaim on the USP V/XP24000 (I think its actually called Discard Zero Data in the GUI) is run as a background task with a low priority so as to have minimum impact on host IO.  Kind of similar to a background Shadow Image copy job or a sparing out operation, only at a far lower priority.  You should not notice a performance hit during the operation.

Just to be sure, I assume that when you say "ZERO Out" you are referring to the Zero Page Reclaim operation performed on the array, and not the way in which you use a host based tool to write zeros to deleted areas of a filesystem (a nifty way to increase the amount of free space you can claim back with a Zero Page Reclaim is to first Zero Out, usually with a secure erase tool, all previously used but now deleted space within a filesystem).  I will probably post on this in the future if you are unsure or there is interest.

Nigel

PS. Glad you like the blog