6 drives, 2x3tb 4x2tb, 3tb show as 2tb WTF

kossboss wrote:

You see Im talking about the extra MD that was created with the oversized disks (md3). You can create a RAID 6 out of 3 block devices....Here is my proof... how many disks are needed for RAID5? 3 right... well then why does our NAS expand the md3 after 2 disks. Look at logs for a setup of 2,2,2,3,3 with RAID 5 you will see that the md3 is expanded with only 2 block devices (the last two 1 TB big portions [2tb to 3tb] of the 3 TB drives are used). Here is a visual of it on readynas.com : http://www.readynas.com/?cat=53 Note that after the 2nd disk upgrade the Volume gets upgraded... Meaning that md3 expanded and is adding its space to the LV /dev/c/c.

That animation is for X-RAID2 single-redundancy.

kossboss wrote:
In other words - its hard to put this into words people can visual To start raid 5 you need 3 disks but to expand raid 5 you only need 2 - with mdadm software To start raid 6 you need 4 disks but to expand raid 6 you only need 3 - with mdadm software

The thing is though in the first case you have single-redundancy. Say a RAID-5 layer of 6x2TB + RAID-1 (that could be converted to RAID-5 when more disks are added) of 2x1TB.

In the second case whilst you could do e.g. RAID-6 layer of 6x2TB + RAID-5 of 3x1TB that wouldn't be dual-redundancy any more, so four disks are required for the volume to expand (regardless of when new md devices are created). When you add a third 3TB disk it might create a degraded RAID-6 layer but in order to maintain dual-redundancy it wouldn't be able to make use of it to expand the volume till a fourth disk is added.

I recently expanded my Ultra 6 volume. I had 6x1.5TB disks installed and I'm using X-RAID2 dual-redundancy. I had to install 4x3TB drives before expansion took place.