VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
Homogenous host configurations
As a best practice, VMware recommends having identical, uniform configurations
across all ESXi hosts participating in a Virtual SAN cluster. While Virtual SAN will
work with different controllers, disks, network interface cards and flash devices in
different hosts, it can lead to imbalance in the deployment of objects and
components across the cluster, which in turn can lead to degraded performance.
Moreover, it can also compromise availability. An imbalanced cluster makes for a
larger fault domain in the cluster. Failure of hosts with larger amounts of storage
can result in higher probability of data unavailability, as there may not be enough
storage capacity remaining in the cluster to rebuild the missing virtual machine
components.
This is the reason why VMware strongly recommends identical configurations on
each host, including number of disks and disk group and cache sizes. There is also a
best practice requirement to have a 1:10 flash cache to capacity layer ratio. This will
be elaborated on in greater detail later in the manual.
Troubleshooting will also be made easier with homogenous host configurations.
A note about multiple controllers and SAS expanders
Virtual SAN supports multiple storage I/O controllers per ESXi host. The maximum
number of capacity layer disks per host is 35 (7 disks per disk group, 5 disk groups
per host). Some controllers have 16 ports and therefore allow up to 16 disks behind
one controller. This means that with just two controllers in one host, a design can
get close to the maximums of 35 disks. However, some controllers only have 8 ports,
so a total of 5 controllers would be needed to reach the maximum (5 flash disks and
35 capacity layer disks).
SAS Expanders are sometimes considered to extend the number of storage devices
that can be configured with a single storage I/O controller. VMware has not
extensively tested SAN expanders with VSAN, and thus does not encourage their
use. In addition to potential compatibility issues, the use of SAS expanders may
impact performance and increase the impact of a failed disk group. Validation of SAS
expanders is still a work in progress.
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