mixing disks of different size

Yes and no. You can mix different size but you would end up with every disk having the same number of bytes which would be the smallest one. You can't add smaller disk afterwards, so add the small disks first.
That's what I understood from the specs of the NAS but never tried.

example :
HDD1=4TO ; HDD2=4TO; HDD3=3TO ==> impossible in Xraid but may be possible in standard raid by having two volumes or maybe in jbod.
HDD1=3To; HDD2=4To; HDD3=4To ==> you will get 6TO in raid 5 (3+3+parity)
HDD1=2To; HDD2=4To; HDD3=4To ==> 4To raid5.

xeltros wrote:
Yes and no. You can mix different size but you would end up with every disk having the same number of bytes which would be the smallest one.

Actually, it's better than that. You do get one layer that's the size of the smallest drive, but if it's possible to add another layer using the extra space on the other drives, you get that too.

xeltros wrote:
You can't add smaller disk afterwards, so add the small disks first.

That is true.

xeltros wrote:
HDD1=3To; HDD2=4To; HDD3=4To ==> you will get 6TO in raid 5 (3+3+parity)

Because of the layering, you will actually get 7TB: One layer that is 3TB (HDD1) + 3TB (HDD2) + parity (HDD3), plus a second layer that is 1TB (HDD2) + parity (HDD3).

       X    X --- 1TB + Parity (second layer)
  X    X    X \
  X    X    X  >- 3TB + Parity (first layer)
  X    X    X /
HDD1 HDD2 HDD3

xeltros wrote:
HDD1=2To; HDD2=4To; HDD3=4To ==> 4To raid5.

Same thing here: 2TB (HDD1) + 2TB (HDD2) + parity (HDD3), plus 2TB (HDD2) + parity (HDD3) = 6TB.

For simplicity, I've written the above as though all the parity is stored on HDD3; that's not actually how it works, but you can visualize it that way for simplicity without affecting the accuracy of the array-size numbers.

Easiest way to calculate the approximate size of a single-redundancy (RAID5) array is to sum the sizes of all drives EXCEPT the largest.

So you can have two volumes without going into flex raid ? I thought X-raid only handled one volume, that's good to know.

xeltros wrote:
So you can have two volumes without going into flex raid ? I thought X-raid only handled one volume, that's good to know.

No, you only get one volume with xraid.

xraid2 will create multiple raid layers to handle disks of different sizes, but they all are part of one volume. The order you add the disks matters. If you have some 1 TB drives, then add some 3 TB drives, then you cannot add 2 TB drives in the future. xraid2 lets you add a new layer on top, but it cannot handle constructing new layers in between. So you can add larger disks than you have already, but not smaller disks.