Problem: X-RAID vertical expansion - swapping in disk smaller than largest in array.

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Readynaspro wrote:

 

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UPDATE: I read somewhere that unless the new swapped-in Hard Drive is not at least as large as the largest drive in the array, that vertical expansion will not be 100%. Something to do with X-RAID being composed of Raid-5 and Raid-1, which I did not undertand. So guessing I am toast and have to factory reset? 

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Help! Please. 

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The limitations of OS-6 expansion are bit a complicated to explain, and have changed a bit over the years.

The general guidance has always been that the new drive either needs to be as large as the biggest drive in the array, or be the same size as the drive it replaces.  It's actually a bit more complicated than that, but those rules are safe guidelines, and never result in sub-optimal space use.

When you expand vertically, the system does create multiple RAID groups that are concatenated together to give you a single volume.  These RAID groups are also reflected in the physical partitions on the disks. 

Let's assume that you

• started with 6 TB drives in the NAS
• with only 8, 10, and 18 TB sizes used later on,
• and that your expansion followed all the rules,

just to help explain. (If that's not the history, your partitions will look different).

Looking at your starting point:

BEFORE:
sda 18TB
sdb 18TB
sdc 6TB
sdd 8TB
sde 8TB
sdf 10TB

If we place the partitions for all disks side by side, we will see this

sda1 sdb1 sdc1 sdd1 sde1 sdf1 (OS)

sda2 sdb2 sdc2 sdd2 sde2 sdf2 (swap)

sda3 sdb3 sdc3 sdd3 sde3 sdf3 (6 x ~6TB RAID-5 md127)

sda4 sdb4          sdd4 sde4 sdf4 (5 x 2TB RAID-5 md126)

sda5 sdb5                            sdf5 (3 x 2TB RAID-5 md125)

sda6 sdb6                                   (2 x 8TB RAID-1 md124)

The mdxxx are the virtual disks associated with each RAID group. 

• md127 (6 x~6 TB) is all the sdX3 partitions, ~30 TB
• md126 (5 x 2 TB) is all the sdX4 partitions,      8 TB
• md125 (3 x 2 TB) is all the sdX5 partitions,      4 TB
• md124 (2 x 8TB) is all the sdX6 partitions,       8 TB

These are all concatenated into your ~50 TB data volume.

When you replaced disk 3 (sdc) the system needs to create the missing partitions.  It can create sdc4 and sdc5 with no problem, but there is not enough space to create sdc6 (it needs to be 8 TB, and your drive only has 6 TB left).   

Going back to the layout grid, you end up with this:

sda1 sdb1 sdc1 sdd1 sde1 sdf1 (OS)

sda2 sdb2 sdc2 sdd2 sde2 sdf2 (swap)

sda3 sdb3 sdc3 sdd3 sde3 sdf3 (6 x ~6TB RAID-5 md127)

sda4 sdb4 sdc4 sdd4 sde4 sdf4 (6 x 2TB RAID-5 md126)

sda5 sdb5 sdc5                   sdf5 (4 x 2TB RAID-5 md125)

sda6 sdb6                                    (2 x 8TB RAID-1 md124)

You end up adding 2 TB to md126, and 2 TB to md125, but md124 can't be expanded.

The simplest solution is to exchange the 16 TB drive for an 18 TB model.

But if you are extremely careful, you can use the linux command line to shrink the sda6 and sdb6 partitions to 6 TB, and create a new 7th partition - ending up with this:

Going back to the layout grid, you end up with this:

sda1 sdb1 sdc1 sdd1 sde1 sdf1 (OS)

sda2 sdb2 sdc2 sdd2 sde2 sdf2 (swap)

sda3 sdb3 sdc3 sdd3 sde3 sdf3 (6 x ~6TB RAID-5 md127)

sda4 sdb4 sdc4 sdd4 sde4 sdf4 (6 x 2TB RAID-5 md126)

sda5 sdb5 sdc5                   sdf5 (4 x 2TB RAID-5 md125)

sda6 sdb6 sdc6                           (3 x 6TB RAID-5 md124)

sda7 sdb7                                   (2 x 2TB RAID-1 md123)

Sandshark sketched out how this can be done here:

• https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/Reducing-RAID-size-removing-drives-WITHOUT-DATA-LOSS-is-possible/td-p/1736125
• https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/How-to-do-incremental-vertical-expansion-in-FlexRAID-mode-using/m-p/1736138

If you decide to attempt this on your own, I strongly recommend making a full backup of the NAS, as a mis-step would result in data loss.