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X-RAID horizontal expansion and storage size
Hi everyone, I've been reading up on X-RAID horizontal expansion for my RN314. I have been running a single 3TB HDD for a few years and recently bought another 3TB HDD to expand storage. I ma...
ilan24 wrote:
But how, if I add another 3TB after that, will I get that full 3TB as well? Wouldn't that mean that I would be sacrificing redundany somewhere?
You wouldn't be sacrificing redundancy.
With 4 disks, you just end up with 3 data blocks and a corresponding parity block. Like the three disk case, the parity block is the sum (actually xor) of the data blocks. You can still compute the missing bytes if any single disk fails - adding up the three data bytes if the parity byte is missing, or subtracting the remaining data bytes from the parity byte if a data byte is missing.
The expansion process includes re-organizing the pattern of data blocks and recomputing all the parity blocks on the disks.
There is one subtilty - redundancy allows you to reconstruct any missing disk, but if the data (or parity) is corrupted, RAID can't figure out which elements of the data is wrong. So it can't repair actual errors.
Thanks Sandshark.
Apologies for this, but I'm still missing something. I understand how adding another 3TB to my existing 2 x 3TB HDDs will get me another 3TB in line with the logic Stephen shared. But how, if I add another 3TB after that, will I get that full 3TB as well? Wouldn't that mean that I would be sacrificing redundany somewhere?
ilan24 wrote:
But how, if I add another 3TB after that, will I get that full 3TB as well? Wouldn't that mean that I would be sacrificing redundany somewhere?
You wouldn't be sacrificing redundancy.
With 4 disks, you just end up with 3 data blocks and a corresponding parity block. Like the three disk case, the parity block is the sum (actually xor) of the data blocks. You can still compute the missing bytes if any single disk fails - adding up the three data bytes if the parity byte is missing, or subtracting the remaining data bytes from the parity byte if a data byte is missing.
The expansion process includes re-organizing the pattern of data blocks and recomputing all the parity blocks on the disks.
There is one subtilty - redundancy allows you to reconstruct any missing disk, but if the data (or parity) is corrupted, RAID can't figure out which elements of the data is wrong. So it can't repair actual errors.
- Thanks so much Stephen and Sandshark - really appreciate the support and this helps a lot!
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