36 | Dell EMC VxRail Appliance Operations Guide
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Figure 14: VM with two replicas and witness
Erasure coding
Erasure coding is an alternative failure tolerance method. Erasure codes provides up to 50
percent more usable capacity than RAID-1 mirroring. Erasure coding is supported on All-Flash
models only.
Erasure coding breaks up data into chunks and distributes them across the nodes in the vSAN
cluster. It provides redundancy by using parity. Data blocks are grouped in sets of n, and for
each set of n data blocks, a set of p parity blocks exists. Together, these sets of (n + p) blocks
make up a stripe. If a drive containing a data block fails, the surviving data blocks (n + p) is
sufficient to recover the data in the stripe.
In VxRail clusters, the data and parity blocks for a single stripe are placed on different ESXi
hosts in a cluster, providing failure tolerance for each stripe. Stripes do not follow a one-to-one
distribution model. It is not a situation where the set of n data blocks sits on one host, and the
parity set sits on another. Rather, the algorithm distributes individual blocks from the parity set
among the ESXi hosts in the cluster.
Erasure coding provides single-parity data protection (RAID-5) that can tolerate one failure
(FTT=1) and double-parity data protection (RAID-6) that can tolerate two failures (FTT=2). The
figures below illustrate the implementations. A single-parity stripe uses three data blocks and
one parity block (3+1), and it requires a minimum of four hosts or four fault domains to ensure
availability in case one of the hosts or disks fails. It represents a 30 percent storage savings
over RAID-1 mirroring. RAID-5 (FTT=1) requires a minimum of four nodes.
Figure 14: Storage object with FTM=Erasure Coding FTT=1
Summary of Contents for VxRail Appliance
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