thanks, I'm getting there.
I thought the data from one head (controller) would be mirrored to the other one . And because of that I was thinking of raid4 and not dp (never had a failure of 2 disks at same time). Your alternative, withou syncmirror, would be then for me one controller with dp (maybe the one with the important data) and the other one raid4, holding just the root volume. The ha hier would be just for a controller failure.
Storage something like that:
contrl 1 - 2 spare disks+ 1 aggr (for root and the important data) with raid dp, 19 disks (2 for parity, 15 for data) ~4,7TB
contrl 2- 1 spare disk+ 1 aggr just with root, raid4- 2 disks (1 parity and 1 for data)- total of 3 disks
And of course the disks are not 300GB, but 450GB
I've forgotten to mention that this filer is going to be used mainly as iSCSI LUNs for wmware, and in view of that, whether it makes sense to use syncmirror at all.
Sorry, it seems that it does not make any sense to use syncmirror with just internal disks (because the aggr will be mirrored inside of a controller)...
I will do without it, like above.