VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
(VMDK) versus a large number of VMs with a small working set (VMDK).
A VMDK size of 1GB for IOmeter should be more than sufficient. However
you might be able to tune the IOmeter working set by choosing a specific
number of sectors to work on, if I remember correctly.
4. Be aware of creating a large working set; you can fill cache and then your
performance will be based on magnetic disk performance due to cache
misses.
5. Be aware of using sustained sequential workloads – these will not benefit
from cache. Use random workloads or a mix of workloads to make full use
of Virtual SAN’s cache layer. Pure sequential workloads are rare, but
important. Virtual SAN may not be best suited to these workload types.
6. Use VSAN Observer to check read cache hit rate and write buffer
evictions/destaging.
7. How many vCPUs do your VMs running IOmeter have versus number of
pCPUs? Scheduling these VMs could be a problem if there is contention,
so check if these VMs are counting up “ready to run” (%READY)
indicating that there is no pCPU resources for them to run on. However
you will also need to balance this with a number of vCPUs that can drive
adequate I/O. Don’t overdo it on number of VMs or number of vCPUs per
VM.
8. Do not set very large Outstanding I/O (OIO) when running lots of VMs
and worker threads. OIO settings between 2 & 8 per VM should be
sufficient.
9. Read/Write mix – start with the traditional “transactional mix” is 70%
read, 30% write. You can also try a 50/50 configuration, which is
considered write intensive. You can later change to a different mix: e.g.
35% read, 65% write and 65% read, 35% write.
10. Set your I/O Size to at least 4KB. However, depending on workload, you
might want something bigger.
11. The observed average latency should be below 5ms, and the max latency
should also be below 50ms. Your numbers may actually be substantially
better, but the conservative numbers are given to cover a wider range of
devices.
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