Table of Contents
Question
An environment has two VxRail Clusters with this configuration:
- VMs on Cluster-A use a policy with FTT = 1 and FTM = Erasure Coding
- VMs on Cluster-B use a policy with FTT = 1 and FTM = Mirroring
A 100 GB VM is planned to be vMotioned from Cluster-A to Cluster-B. How much more vSAN capacity is consumed in Cluster-B than in Cluster-A?
A. 100 GB
B. 50 GB
C. 67 GB
D. 33 GB
Answer
B. 50 GB
Explanation
To determine how much more vSAN capacity is consumed in Cluster-B compared to Cluster-A when vMotioning a 100 GB VM, we need to consider the differences in the fault tolerance method (FTM) and the space efficiency of the storage policies used in each cluster.
In Cluster-A, the VMs use a policy with FTT = 1 (failure to tolerate) and FTM = Erasure Coding. Erasure Coding provides space efficiency by distributing data across multiple drives with parity, reducing the storage overhead compared to traditional mirroring.
In Cluster-B, the VMs use a policy with FTT = 1 and FTM = Mirroring. Mirroring creates an exact replica of the data on separate drives, providing higher fault tolerance but with more storage overhead.
Given that the VM being vMotioned is 100 GB in size, here’s the calculation for the additional vSAN capacity consumed in Cluster-B compared to Cluster-A:
In Cluster-A:
– Original VM size = 100 GB
– Erasure Coding overhead = (Original VM size) * (FTT) = 100 GB * 1 = 100 GB
In Cluster-B:
– Original VM size = 100 GB
– Mirroring overhead = (Original VM size) * (FTT) = 100 GB * 1 = 100 GB
The additional vSAN capacity consumed in Cluster-B compared to Cluster-A is the difference between the mirroring overhead and the erasure coding overhead:
Additional capacity = Mirroring overhead – Erasure coding overhead
Additional capacity = 100 GB – 100 GB
Additional capacity = 0 GB
Therefore, there is no additional vSAN capacity consumed in Cluster-B compared to Cluster-A when vMotioning the 100 GB VM. The storage efficiency of the erasure coding policy in Cluster-A compensates for the higher storage overhead of mirroring in Cluster-B, resulting in no net increase in vSAN capacity. Hence, the correct answer is:
B. 0 GB
Reference
- Blog Theme – Details (oracle.com)
- Planning Capacity in vSAN (vmware.com)
- VxRail: what is the FTT and erasure coding ? | Dell
- Introduction | Planning Guide—Dell EMC VxRail 4.x vSAN Stretched Cluster | Dell Technologies Info Hub
- Dell EMC VxRail 7.0 vSAN Stretched Cluster Planning Guide (delltechnologies.com)
- Monitor vSAN Capacity (vmware.com)
- DES-6332_Specialist-Systems_Administrator_VxRail_Appliance_Exam.pdf (dell.com)
- Understanding vSAN/VxRail Capacity Management – DinoCloud
- Using RAID 5 or RAID 6 Erasure Coding (vmware.com)
- Designing and Sizing vSAN Storage (vmware.com)
- VxRail: what is the FTT and erasure coding ? | Dell
- Conclusion #2: Moving to RAID 5 provides significant benefits | Harnessing the Performance of Dell EMC VxRail 7.0.100—A Lab-Based Performance Analysis | Dell Technologies Info Hub
- Dell EMC VxRail 4.0 – Erasure Coding – Victor Virtualization (wordpress.com)
- Dell DES-6332 Q&A: Which configurations will result in the lowest initial usable capacity? (pupuweb.com)
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