Raid5 is not dead yet (<a href="https:&#x2F;&#x2F;www.cafaro.net&#x2F;2014&#x2F;05&#x2F;26&#x2F;why-raid-5-is-not-dead-yet&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.cafaro.net&#x2F;2014&#x2F;05&#x2F;26&#x2F;why-raid-5-is-not-dead-yet...</a>). The problem with failures during rebuilds is overblown, IMO. Manufacturer quoted URE failure rate (probability of failure to read) is overstated - instead of 1×10^14, they are mostly like 1×10^15 or higher. 
Full disclosure: we&#x27;re actually doing erasure coding in HDFS over Raid5 on servers (double insurance - if the raid array goes down, we can recover from other servers in HDFS). But our expectation for 6x4TB arrays is not for a 70%+ chance of a URE during a rebuild, rather a couple of percent. With ZFS or btrfs, it won&#x27;t actually matter for us, as we&#x27;ll only lose a block on a URE- that we can recover from the rest of the cluster.