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Understanding X-RAID and capacity
I'm thinking of upgrading from an almost full ReadyNAS Duo (with two 2GB disks) to an RN10400. I've got a spare 2GB disk and plan to buy an additional 4TB disk. Given these three 2GB disks and th...
geojay wrote:
I don't understand your logic I'm afraid. I understand that the 4TB drive will only function as a 2TB drive in this configuration so let's treat it as a 2TB drive and then we have four 2TB drives or 8TB total. I'm assuming everything is redundant so does that not give us 4TB of effective capacity?
Nope.
With this disk configuration XRAID uses RAID-5. Here's a (somewhat simplified) picture of how RAID-5 redundancy works. It's a math trick.
Imagine 3 blocks of data - A, B, C. These blocks are stored on three of the disks.
Now compute P=A+B+C, and store that in the corresponding slot of the fourth disk.
If any disk fails, you can compute the missing block from the corresponding blocks on the other three disks.
A = P-B-C
B = P-A-C
C = P-A-B
P = A+B+C
So you are protected from a single disk failure. The overhead is 25% in this case, not 50%.
XRAID is a more sophisticated when you have mixed disk sizes. But even with mixed disk sizes, capacity = (sum of all drives)-largest
Capacity = (sum of all drives)-largest
That is 6 TB with 3x2TB+4TB. 2 TB of the 4TB drive won't be used until you upgrade another drive to 4 TB.
I don't understand your logic I'm afraid. I understand that the 4TB drive will only function as a 2TB drive in this configuration so let's treat it as a 2TB drive and then we have four 2TB drives or 8TB total. I'm assuming everything is redundant so does that not give us 4TB of effective capacity?
Thanks!
StephenB wrote:Capacity = (sum of all drives)-largest
That is 6 TB with 3x2TB+4TB. 2 TB of the 4TB drive won't be used until you upgrade another drive to 4 TB.
geojay wrote:
I don't understand your logic I'm afraid. I understand that the 4TB drive will only function as a 2TB drive in this configuration so let's treat it as a 2TB drive and then we have four 2TB drives or 8TB total. I'm assuming everything is redundant so does that not give us 4TB of effective capacity?
Nope.
With this disk configuration XRAID uses RAID-5. Here's a (somewhat simplified) picture of how RAID-5 redundancy works. It's a math trick.
Imagine 3 blocks of data - A, B, C. These blocks are stored on three of the disks.
Now compute P=A+B+C, and store that in the corresponding slot of the fourth disk.
If any disk fails, you can compute the missing block from the corresponding blocks on the other three disks.
A = P-B-C
B = P-A-C
C = P-A-B
P = A+B+C
So you are protected from a single disk failure. The overhead is 25% in this case, not 50%.
XRAID is a more sophisticated when you have mixed disk sizes. But even with mixed disk sizes, capacity = (sum of all drives)-largest
OK, 'math trick', I'll leave it there... :)
So is to say that if I eventually populate it with four 4TB drives (I'm not actually sure what the maximum drive size is after looking at the specs, it just says 16TB maximum, I assume that's actual storage distributed across the four bays?!) then that'll give me 12TB of effective storage?
Thanks for your assistance!
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