ReadyNAS 104 Convert X-Raid to Flex Raid - will it expand space for data?

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mickle026 wrote:



So to clarify, 3x 2tb disks in x-raid becomes 4tb raid 5 with data recovery possible.

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3x2 tb xraid is raid-5.  If you change to flexraid it remains raid-5.  There is protection from loss of a single disk.

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mickle026 wrote:


So what does 4x 2tb become - 6tb?

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Yes.

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mickle026 wrote:



My next question will be if 4x 2tb x-raid become 6tb how is it possible to recover 6 tb of data from 3 drives if 1 fails

Are you asking how RAID-5 works?  

If so - 

Data is organized as "stripes" across the three drives.  For each stripe, there are two data blocks (on different drives), and a parity block on the third drive.

The parity block is the logical xor of the two data blocks.  You can think of this as the sum of the two data blocks (xor is analogous to addition in a Galois field - which is used in RAID-5 and RAID-6)..

So if D1 + D2 = P, then if one of the data blocks is missing, it can be reconstructed as 

D1 = P - D2 or

D2 = P - D1.

In order for this to work, the NAS needs to know which block is missing.  When you insert a new disk, it of course does know this, and it reconstructs all the data on it (including P blocks) from the other two.

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mickle026 wrote:
 ...worse still if 2 fail?

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RAID-5 doesn't protect against two disk failures.   RAID-6 will - and if you wish you could enable RAID-6 if you have 4 disks of the same size.  With 4x2TB you'd only get 4 TB of data volume, and with the RN104 the write performance would be quite slow.

In any event, it is important to understand that RAID protection can and does fail.  So you still need backups even with RAID.