VSAN-Troubleshooting-Reference-Manual

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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. V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 2 5 8
Diagnostics and Troubleshooting Reference Manual – Virtual SAN Using esxtop for performance monitoring In version 6.0, esxtop now has a new, limited view for Virtual SAN. A new option has been added (x) to esxtop: By default, this view will display the 3 different roles of the Distributed Object Manager (DOM) that exist on the host. The roles are client, owner and component server. The statistics that are displayed by default show reads and writes and include IOPS, bandwidth, average latency and latency standard deviation. There is one additional field that is not displayed by default. This is the “recovery write statistics” and displays how much writing on behalf of a remediation task that is doing a rebuild of components in the Virtual SAN cluster. V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 2 5 9
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