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Troubleshooting Mindset Series #021: When Multiple VMs Become Slow at the Same Time
  

rizzuan Lv2  Posted 2026-Oct-01 10:54

A single slow VM can often be investigated from the VM or application layer.
But what if several VMs become slow at almost the same time?
That changes the troubleshooting direction.
Instead of investigating each VM independently, I would first ask:

What do these VMs have in common?

1. Check the ScopeStart by identifying how many VMs are affected.
  • One VM only → investigate VM/application/guest OS
  • Several VMs on the same host → check host resources and local path
  • Several VMs across different hosts → check shared HCI resources
  • Many VMs across the cluster → investigate cluster-wide components

The scope can quickly tell us where to look next.

2. Look for Shared ResourcesIf multiple VMs are affected, check resources that they share:
  • HCI storage
  • Storage latency
  • IOPS and throughput
  • Disk/SSD health
  • Storage contention
  • Network path
  • Host resource utilization
  • Cluster alerts or background tasks

A problem affecting a shared layer can make multiple VMs appear slow at the same time.

3. Check Time CorrelationAsk:
Did all affected VMs become slow at approximately the same time?
If yes, compare the timeline with:
  • HCI alerts
  • Storage events
  • Disk/SSD activities
  • Rebuild or background tasks
  • VM migration
  • Backup activities
  • Network events
  • Recent configuration changes

Time correlation can help separate a common infrastructure event from unrelated VM issues.

4. Compare Affected vs Healthy VMsDon't only look at the slow VMs.
Find a healthy VM and compare:
Affected VM → Host → Storage → Network path
against
Healthy VM → Host → Storage → Network path
The differences can be more useful than the absolute numbers.

Common MistakeA common mistake is troubleshooting every affected VM separately.
If ten VMs become slow at the same time, it may not be ten separate problems.
There may be one shared problem affecting ten VMs.

Troubleshooting MindsetWhen multiple VMs are affected:
Don't start with “Why is this VM slow?”
Start with:
“What do these affected VMs have in common?”
Then trace the shared path and correlate it with the time the problem started.

Key TakeawayOne affected VM → investigate the VM.
Multiple affected VMs → investigate what they share.
Sometimes the fastest way to find the root cause is not to look deeper into each VM, but to step back and look for the common dependency.
#Sangfor #HCI #Troubleshooting #Virtualization #Storage #TroubleshootingMindset

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