RN104 /// RAID1 /// 4xHDD /// 1 VOLUME

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j-o-h-a-n wrote:

A).  Two parallel RAID 1 volumes 2x4TB + 2x4TB (= approx. 7,5TB?)

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Volume size for each volume would be 4 TB, so 8 TB total.  The NAS (like Windows) reports size in TiB (1024*1024*1024*1024 bytes), and 8 TB is the same as ~7.25 TiB.

A variant is to set up a single 2x8TB to start with, and add a second volume later on.  Four 4 TB drives are about the same cost as two 8 TB, and with RAID-1 you get the same amount of storage. But you'd also have 2 free slots, so the first expansion wouldn't require replacing working disks.

With this approach, you'd create some shares on each volume.  You'd see the full share list from Windows, and normally users wouldn't know (or care) which share is on which volume.  As administrator, you would need to keep a reasonable amount of free space on both volumes.  You sometimes could shift a share from one volume to the other to roughly balance free space.

Expansion of the two volumes could be done independently, and you would need to upgrade both disks in the volume to the larger size in order to expand.

One benefit of RAID-1 is that data recovery is fairly simple, since the data is mirrored on both disks.  So each disk has a complete copy of the data.

That said, RAID is not enough to keep your data safe, so you should also put a backup plan in place for your data.  Using one or more USB drives is a popular option.

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j-o-h-a-n wrote:

B).  One RAID 5 volume 4x4TB (approx. 12 TB?)

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If you choose this mode, just stick with the default X-RAID - it manages the RAID setup for you.  With "classic" RAID-5, all the disks would need to upgraded to a larger size when it is time to expand.  X-RAID supports unequal drive sizes, and would allow you to upgrade less expensively.  Note X-RAID uses standard software RAID underneath, it is just a management package.

Size would be 12 TB (10.9 TiB) - the general capacity rule is "sum the disks and subtract the largest". 

This approach gives you the most capacity possible with single redundancy (handling one disk failure with no data loss).

With X-RAID, the first expansion would require upgrading a pair of disks - for example, expanding to 2x8TB+2x4TB.  Capacity would then grow to 16 TB.  You could then upgrade a single disk later on (3x8TB+4TB), increasing capacity to 20 TB.

Data Recovery (if ever needed) would require at least three disks to be accessible at the same time, so it is more complicated than A).  That said, if you put a good backup plan in place, you aren't likely to ever need data recovery.  And no matter what approach you take, data recovery is expensive, and not always successful.