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I have a QNAP TS-412 NAS and used two disks so far. I decided to upgrade to four disks and now have options with respect to RAID. I am looking for maximum security against failure of disks (preferably the system should still survive with two disk failures, preferably even if those two disks are ANY disks in the array, but that might not be feasible). The NAS needs a solid good performance for reading, but not that much for writing data (i.e. data is relatively stable). I do media steaming (which is time critical in terms of reading to not have lags) and I do backups (which are not time critical at all, but require safety and integrity).

Utilisation is irrelevant to me, I just want that I have my data very save and I accept 50% Utilisation as the price.

Would it be advisable to leave the existing 2-disk RAID1, add the extra two disks and then create ANOTER RAID1 with these new disks?

Or would it be better to have a RAID10 instead, and combine the four disks that way into a single volume?

Two RAID1 arrangements can only have one disk per RAID1 fail at any time. Is this the same for a RAID10? Or are there additional advantages that a RAID10 provides that a double RAID1 would not?

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RAID 10 is just stripes over mirrors. That means for every mirror pair only one drive may fail. You also lose 50% of the storage. I don't know how your NAS handles two RAID 1 arrays but it might be a bit faster to use RAID 10.

If you want better fault tolerance you could use RAID 6. There you still have 50% capacity loss but any two drives may fail.

RAID 10 is probably the fastest and RAID 6 the most secure.

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RAID 10 differs slightly from 2x RAID 1: it stripes the two mirrored subarrays, improving the performance, but it relies on availability of both subarrays. In a nutshell, RAID10 will fail totally when any subarray fails completely (both disks).

With dual RAID 1, the second array will stay online even if both disks in the first array fail.

If you require the array to survive failures of any two disks you need RAID 6 as Christopher has already pointed out. Due to the necessary computing overhead and the performance of the NAS's Marvel CPU, you should expect a significant performance hit on writes though.

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