I have four similar 2TB discs in a 2x2 mirror array. If I transfer external data to the pool it seems that each of the four disks is receiving data at the same rate so presumably each disk will receive the same amount of data (rate x time).

If we call the disks 1, 2, 1A and 2A where 1 and 2 could be called the primary pair it makes sense that 1A will receive the same amount of data as 1 and 2A will receive the same amount of data as 2 so 1A and 2A will be exact mirrors of 1 and 2.

To achieve this I imagine the data is sent in constant size packages first to disk 1 and 1A and then the next package goes to disks 2 and 2A - rinse and repeat until there is no more data. If so the write sequence would be 1 then 1A followed by 2 and 2A so the data would be sort of striped between 1 and 2 with a mirror image on 1A and 2A.

This would seem to imply that if any one disk fails then you could rebuild it from its corresponding paired disk. If you lost, say, disks 1A and 2A you could still rebuild the system from disks 1 and 2, but if you lost both 1 and 1A or both 2 and 2A you are dead in the water.

Is that the way things work?

Thanks.