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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
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!
geojay wrote:
So is to say that if I eventually populate it with four 4TB drives ... then that'll give me 12TB of effective storage?
Yes.
You can replace a drive with one of identical size, or with one that is at least as large as the biggest one current in the array. You can't install a smaller size.
If a slot is empty, add a drive that is at least as large as the biggest one in the array.
Also (in case you are not aware), drive manufacturers use TB units (1000*1000*1000*1000 bytes); Windows and ReadyNAS use TiB units (1024*1024*1024*1024 bytes). 1 TB = .9 TiB. So a 12 TB volume will be reported as 10.9 TiB. Unfortunately both Windows and ReadyNAS use the "TB" label for TiB, which makes this unit mess even more confusing.
geojay wrote:
...I'm not actually sure what the maximum drive size is after looking at the specs, it just says 16TB maximum...
There is no known limit in the firmware. However, if you are using xraid, then 4 TB drives are a good ceiling for the RN104. Bigger drives will work (I have a 6 TB and an 8 TB in my RN102), but RAID sync times get long. RAID sync isn't a problem for me, since I am using jbod in the RN102 (it is a backup of my main NAS, so I opted for capacity instead of failure protection).
FWIW, if your budget will stretch to an RN204 then I'd suggest going for that instead of the RN104. It is quite a bit faster.
FWIW, if your budget will stretch to an RN204 then I'd suggest going for that instead of the RN104. It is quite a bit faster.Ah, that's food for thought as there's not a massive difference in price...
Can you point me at any meaningful comparisons of the two devices? I'm not managing to find anything that lets me compare one with the other...
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