Currently I have a linux storage box and server with 4x750gb harddrives in raid-5 with ext3. I have ordered 3x1.5tb disks to upgrade this. Here is my planned upgrade:

Backup:

1. Format the 1.5 tb disks
2. Copy all data from the raid-5 disks to the 1.5tb disks
3. Destroy the raid-5 array.

New setup:

1. Create a VirtualBox system and install Nexenta (OpenSolaris + ubuntu) on it.
2. Create a zfs pool with zraid1 with the 4 750gb disks.
3. Copy from 1.5tb disks to the virtualbox zfs pool
4. Format the 1.5tb disks.
5. Replace 3 off the 750gb disks with 1.5tb disks.
6. Reuse the 750gb disks elsewhere.

The reason I wish to use one 750gb disk is since I can't grow the disk count in a raidz array, and this gives me the option off replacing that disk later for an extra 750gb storage.

Would the ZFS performance be good running through virtualbox? Or will the performance overhead be too large? Will I get 1.5tb+1.5tb+750gb storage on the zraid? Or just 750gbx3 until all disks are 1.5tb?