37 | Dell EMC VxRail Appliance Operations Guide
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Dual parity saves as much as 50 percent capacity over RAID-1. It uses four data blocks plus
two parity blocks (4+2) and requires a minimum of six nodes (as shown below).
Figure 14: Storage object with FTM=Erasure Coding FTT=1
The figure below compare the usable capacity for mirroring and erasure-code fault tolerance
method. As you can see erasure coding can increase usable capacity up to 50 percent
compared to mirroring.
Figure 14: Usable capacity savings for Erasure Coding
With erasure coding, in the event of a drive failure, backend activity increases. During a rebuild
operation, a single read from the VM requires multiple reads from disk and additional network
traffic, since the surviving drives in a stripe must be read to calculate the data of the failed
member. This additional IO is the primary reason why only all-flash VxRail configurations use
erasure coding. The rationale is that the speed of flash disks compensate for the additional
overhead.
Note: with VxRail 4.5, rebuild rate is configurable and this activity can be throttled to minimize
the impact to other workloads that run on the cluster. However by throttling resynchronization,
the time that the data is exposed in the event of another drive failure increases.
Summary of Contents for VxRail Appliance
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