ReadyNas Pro 4 expansion

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TMcL wrote:

 

I thought I set it up with redudundancy because even though I had 4x1T disks, I only ever had about 2T. So I thought that my disks were duplicated.

But now I have the error below. Does it just mean I have to replace disk 1 and then i will get my redundancy back? 

 

Configuration:  RAID Level X-RAID2, 4 disks  

Status:  Not redundant. A disk failure will render this volume dead.        

 

RAID Disks:

Ch 1 : WDC WD1002FBYS-02A6B0 [931 GB]      had not-green dot next to it

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Replacing disk 1 will restore your redundancy.

4x1TB X-RAID2 uses standard RAID-5.  So the capacity would be 3 TB.  However, the Web UI actually reports the size in TiB (1024*1024*1024*1024 byte = 1 TiB) - so it should report ~2.7 TiB.

FWIW, the disks aren't duplicated - the redundancy is provided in a different way.  The disks are organized into data blocks and parity blocks - for example you'd have data block A on disk 1, data block B on disk 2, data block C on disk 3, and parity block P on disk 4.

The partity block P is computed from A, B, C.

P=A+B+C

The actual partity block construction doesn't use addition (it uses XOR), but you can think of it as addition.  Every time you update one of the data blocks, the NAS recomputes the associated parity block.

Then if a disk fails or is replaced, you can reconstruct the missing block using one of the equations below:

A=B+C-P
B=A+C-P
C=A+B-P
P=A+B+C

In this example the first equation would apply.  Note that the parity block isn't always on disk 4, the pattern is rotated so that some partity blocks are on each disk.

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TMcL wrote:

Also, I am hitting my storage limit. Can I replace with (supported)  4x3T drives (one by one starting with goner)? I THINK that will keep me under the 8T max expansion limit. 

 

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You should be fine. 4x3TB gives you a 9 TB volume (~8.2 TiB).  So even if you started with a single 1 TB drive you'd still be under the 8 TiB expansion limit (expanding about ~6.4 TiB ).