VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
Obviously, if congestion begins to occur, higher latencies will also be observed.
Sustained congestion is not normal and in most cases congestion should be close to
zero. Smaller values for congestion are better.
What is outstanding I/O (OIO)?
When a virtual machine requests for certain I/O to be performed (reads or writes),
these requests are sent to storage devices. Until these requests are complete they
are termed outstanding I/O. This metric explains how many operations are waiting
on Virtual SAN, SSD cache, I/O controller and disks. Large amounts of outstanding
I/O can have an adverse effect on the device latency. Storage controllers that have a
large queue depth can handle higher outstanding I/Os. For this reason, VMware only
qualifies storage controllers with a significantly high queue depth, and does not
support controllers with low queue depth values. Smaller numbers for OIO are
better.
What is latency standard deviation (stddev)?
The latency stddev (standard deviation) graph gives an idea of how wide the latency
spread is at any given point. It is a measurement of the standard deviation of all
latency times, which shows the variability of latency across all I/O requests. Lower
standard deviation implies that the latency is predictable and well bounded
(workload is running in a steady state). Higher values indicate that latency is
fluctuating heavily and possibly warrants further investigation.
What to look for in the VSAN Client view?
If this view shows any unexpected performance, in other words, any of the charts
are underlined in red, the next step is to drill down further to understand where
performance issues may be coming from.
It is important to understand that due to the distributed nature of VSAN, any host
may access data from disks on any of the other hosts in the cluster. Therefore any
host in the VSAN cluster may cause any performance issue seen in the VSAN Client
view. The next steps will be:
To check the ‘VSAN disks’ view to learn more about any bottlenecks that
might be attributed to the disks (either magnetic disk or flash device/SSD).
Check the ‘VMs’ centric view to see how individual disks and hosts are
contributing to a virtual machines performance.
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