Help me understand redundancy

TrueNAS General
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I finished creating a RAIDZ2 pool and was curious to know about the redundancy and how does it actually work.

If i’m not wrong, RAIDZ2 requires 4 disk and provides upto 2 disk failures. Let’s say i’m using 8x10TB disks. Out of which i’ll get 6x10TB as 2 will be used for partity. Roughly, 6x10TB = 60TB of raw capacity and let’s say the pool has 40TB data. So, how come just 2 parity disks which just has 20TB raw capacity protects the data when it’s actually 40TB of data?

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You are using two disks of space for parity, not two specific disks. It’s spread over all the disks in the VDEV

White Papers | TrueNAS - Open Enterprise Storage Look for Pool layout whitepaper at bottom

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…and while ZFS’ implementation differs considerably from earlier versions of RAID; the principles of parity RAID go back decades. If you want to understand the details of how it works, old documentation on RAID 5 and RAID 6 should also be relevant.

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