35 | Dell EMC VxRail Appliance Operations Guide
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Choosing a failure tolerance method (FTM)
To protect virtual machine storage object on a vSAN datastore from drive failure, VxRail
Appliance supports both mirroring and erasure coding
failure tolerance methods
(FTM).
Mirroring (RAID-1) provides better performance and consumes less memory and network
resources, but uses more disk space. Erasure coding (RAID5/6) uses parity-based protection
and provides more usable capacity, but consumes more CPU and network resources.
FTM for a virtual machine storage object is configured as a SPBM rule and works with the
failures to tolerate
(FTT) rule. If FTM= mirroring and FTT=1, two replicas of data are maintained.
If FTM=mirroring and FTT=2, there are three replicas of data. If FTM=erasure coding and
FTT=1, a RAID-5-like data protection is used. If FTM= erasure coding and FTT=2, a RAID-6-like
data protection is used.
The appropriate failure tolerance method depends on the workload characteristics and VxRail
system configuration. Generally, heavy write-intensive workloads are better suited for mirrored
FTM and read-intensive workloads work well with erasure coding. Your Dell EMC representative
can help you determine the optimal
failure tolerance method
by modeling the characteristics of
your workload against a specific VxRail system configuration.
A VxRail system can have multiple SPBM rules allowing the flexibility to configure different FTM
rules for different workloads that run on the same system. The following sections review how the
FTM and FTT rules impact the configuration of storage objects within vSAN.
Mirroring
Mirroring is supported with both hybrid and All-Flash VxRail models. If FTM=mirroring and
FTT=1, two replicas of data are maintained. If FTM=mirroring and FTT=2, there are three
replicas of data. In addition, vSAN uses the concept of a witness. When determining if a
component remains online after a failure, more than 50% of the components that make up a
storage object must be available.
Witnesses are components that contain only metadata. Their purpose is to serve as tiebreakers
when determining if a quorum of components is online in the cluster. If more than 50% of the
components that make up a virtual machine’s storage object are available after a failure, the
object remains online. If less than 50% of the components of an object are available across all
the nodes in a VSAN cluster, that object is no longer available. Witness prevents “split brain
syndrome” in a vSAN Cluster
.
The figure below illustrates a four-node cluster and a virtual machine with a FTM=mirroring and
a FTT=1. Note the two replicas and the witness.
Summary of Contents for VxRail Appliance
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